Why Would Only One Tonsil Be Swollen?

A single swollen tonsil almost always traces back to a localized infection, most commonly a peritonsillar abscess or peritonsillar cellulitis, though tonsil stones, chronic low-grade infections, and even benign growths can produce one-sided enlargement. The reassuring reality is that cancer accounts for a very small fraction of cases, but it is the possibility that makes the symptom worth understanding. The causes range widely enough that context matters: your age, how quickly the swelling appeared, whether it hurts, and what other symptoms tag along all shift the likely explanation.

Peritonsillar Abscess and Cellulitis

The single most recognizable cause of a dramatically swollen tonsil on one side is a peritonsillar abscess, sometimes called quinsy. This happens when an infection breaks past the tonsil itself and forms a pocket of pus in the tissue just behind it. The hallmark is a visible bulge on one side of the soft palate, often pushing the uvula toward the opposite side. Symptoms tend to come on fast and feel severe: intense throat pain that’s worse on one side, difficulty opening your mouth, a muffled or “hot potato” voice, painful swallowing, and ear pain on the same side.¹

When a doctor suspects quinsy, they typically attempt a needle aspiration. If pus comes out, the diagnosis is confirmed. If no pus is found, the condition is reclassified as peritonsillar cellulitis, which is essentially the same type of spreading infection without a walled-off abscess.¹ The distinction matters for treatment: an abscess usually needs drainage, while cellulitis can often be managed with antibiotics alone. Both cause obvious unilateral swelling, and both tend to resolve once treated. The key clue that you’re dealing with one of these rather than something more chronic is speed. Peritonsillar infections develop over days, not weeks, and the pain is hard to ignore.

Bacterial Infections That Favor One Side

Standard sore throats from strep or viral infections usually affect both tonsils more or less equally, so when only one side swells, the bacterial picture is sometimes a bit different. One organism worth knowing about is Fusobacterium necrophorum, a bacterium that colonizes the tonsils more often than most people realize. In one study of patients undergoing tonsillectomy, F. necrophorum was found in about 28% of throat swabs at baseline, and among patients who had an active tonsillar infection at the time, six out of ten cultured positive for it.² F. necrophorum matters because it’s the main culprit behind Lemierre syndrome, a rare but serious complication where a throat infection leads to an infected blood clot in the jugular vein. That’s an extreme outcome, but the bacterium’s tendency to cause aggressive, sometimes lopsided tonsillar infections is part of why unilateral swelling with fever and neck stiffness warrants prompt medical attention.

Group A streptococcus and other beta-hemolytic streptococci can also cause asymmetric swelling if the infection progresses unevenly or if one tonsil happens to have deeper crypts where bacteria take hold more readily. Most routine strep throat doesn’t produce dramatic one-sided enlargement, but when it does, it may be heading toward a peritonsillar complication.

Tonsil Stones That Get Large Enough to Notice

Tonsil stones, or tonsilloliths, are hardened clumps of debris that form in the folds and pockets of the tonsils. Most are tiny, white or yellowish, and either fall out on their own or cause nothing more than occasional bad breath. But in rare cases they grow large enough to make the tonsil look significantly swollen on one side. One case report documented a tonsil stone measuring 5.2 by 2.5 centimeters that initially looked like a peritonsillar abscess on examination.³ That’s an extreme example, but it illustrates the point: a stone doesn’t have to be enormous to make the area feel full, tender, or asymmetric. Symptoms of a sizeable stone include a foreign-body sensation in the throat, mild pain or heaviness, and persistent bad breath that doesn’t respond to normal oral hygiene.

Tonsil stones are more of a nuisance than a danger. They form because the tonsils naturally have deep crypts that can trap food particles, dead cells, and bacteria. Some people’s anatomy makes them more prone to this than others. If one tonsil has deeper or more numerous crypts, stones tend to accumulate there, producing one-sided swelling that waxes and wanes rather than developing acutely.

Actinomycosis and Other Slow-Burning Infections

Not every infection announces itself with acute pain and fever. Actinomycosis is a chronic, slowly progressive bacterial infection caused by Actinomyces species, which are normal inhabitants of the mouth. Under certain conditions, they invade tonsillar tissue and produce a growing mass that can look alarming. Case reports describe actinomycosis causing massive unilateral tonsillar enlargement that mimics a tumor on examination.⁴ Multiple case series have confirmed that Actinomyces can cause enough one-sided growth to create obvious tonsillar asymmetry.⁵

Actinomycosis is treatable with prolonged courses of antibiotics, but it has to be diagnosed first, and that’s the challenge. Because the swelling develops gradually and may not be particularly painful, it’s easy to overlook or to mistake for something more worrisome. In one large tonsillectomy series, actinomycosis was found in about 8% of pediatric specimens.⁶ It’s not rare, just rarely suspected before the tissue is examined under a microscope.

Granulomatous Diseases

A less common but clinically important group of causes involves granulomatous inflammation, where the immune system walls off perceived threats into clusters of inflammatory cells called granulomas. In a series of 22 tonsillar granuloma cases, the underlying causes included sarcoidosis in eight patients, tuberculosis in three, Hodgkin’s lymphoma in two, toxoplasmosis in one, squamous cell carcinoma in one, and no identifiable cause in seven.⁷ The mix of benign and malignant causes in that list explains why granulomatous tonsil swelling gets taken seriously. Sarcoidosis and tuberculosis each produce chronic, often painless enlargement of one tonsil that can persist for weeks or months without an obvious explanation. Neither can be diagnosed by appearance alone; tissue biopsy and sometimes specialized cultures or imaging are needed.

Benign Growths

Tumors don’t have to be cancerous to make a tonsil look swollen. Lymphangiomatous polyps are benign growths that arise from lymphatic tissue within the tonsil and present as a mass protruding from the tonsillar surface. They’ve been reported in both adults and very young children.⁸⁹ These polyps consist of a fibrovascular core covered by normal-looking squamous epithelium, with lymphatic channels running through them. Histological examination shows no cellular atypia, which is the pathologist’s way of saying the cells look normal and not pre-cancerous.¹⁰ Once removed, they don’t come back, and no further treatment is needed.

Other benign possibilities include papillomas, fibromas, and lipomas, though all are uncommon. The practical point is that a painless lump on one tonsil isn’t automatically cancer. But because it’s impossible to tell benign from malignant by sight, any persistent unexplained mass warrants evaluation.

When It Could Be Cancer

This is the concern that drives most people to search for answers about a single swollen tonsil, so it deserves a clear-eyed look at the numbers. Among head and neck lymphomas, the tonsil is actually a common site, with roughly 40 to 50% of lymphomas in the Waldeyer ring (the ring of lymphoid tissue at the back of the throat) arising from the tonsils.¹¹ Diffuse large B-cell lymphoma (DLBCL) is the most common type. Squamous cell carcinoma, often linked to HPV infection, is the other major malignancy seen in tonsils.¹²

That said, the overall likelihood of malignancy in someone who simply has one tonsil bigger than the other, with no other worrisome signs, is very low. A recent meta-analysis calculated that the positive predictive value of tonsillar asymmetry alone, meaning the chance that asymmetry actually means cancer, was about 4.3%. But when the analysis excluded patients with additional red flags like swollen neck lymph nodes, the probability dropped to just 0.16%.¹³ In other words, if you have no neck lumps, no weight loss, no night sweats, and no other suspicious findings, the odds strongly favor a benign explanation. Conversely, when additional high-risk features like lymphadenopathy were present, the probability of malignancy jumped to about 38.5%.¹³

The negative predictive value in that same analysis was 99.8%, meaning that when tonsils appear symmetric, cancer is almost certainly not present. The reassurance here is real but comes with an asterisk: some malignancies, particularly lymphoma in children, can present with a painless, slowly growing tonsil in an otherwise well-looking patient. In one pediatric case, an 11-year-old had a painless, hardened nodule on one tonsil that had been growing for a year before anyone investigated. Tonsillectomy revealed diffuse large B-cell lymphoma.¹⁴

How Age Changes the Picture

Your age significantly shifts what a swollen tonsil is likely to mean. In children, the tonsils are at their most immunologically active and naturally vary in size, sometimes quite a lot, without any disease being present. A large study comparing pediatric and adult tonsillectomy specimens found that among children, only 0.13% had malignancy (a single lymphoma case), while the rest showed chronic inflammation, reactive tissue, or benign lymphoid overgrowth. In adults, the malignancy rate was 2.04%, roughly fifteen times higher.⁶ The researchers noted that in every case where malignancy was found, there had been at least one suspicious clinical feature before surgery.

For adults, the risk climbs further with age. In a study of 87 patients with asymmetric but otherwise normal-appearing tonsils, two cases of malignancy were found, both in patients over 50.¹⁵ This doesn’t mean every adult with one big tonsil needs emergency surgery, but it does mean that the threshold for investigation is lower the older you are. A pediatrician might reasonably watch and wait with an otherwise healthy child; an ENT specialist seeing a 55-year-old with the same finding will likely want a biopsy.

Red Flags That Change the Urgency

Tonsillar asymmetry by itself is common and usually benign, a point that bears emphasis because the anxiety it generates is often disproportionate to the actual risk. The clinical protocols used in primary care distinguish between low-risk asymptomatic asymmetry and high-risk cases needing urgent referral based on a handful of specific warning signs.

Features that raise the index of suspicion include:

Neck lumps: swollen, firm, or non-tender lymph nodes on the same side as the enlarged tonsil, especially if they persist for more than two to three weeks
Rapid growth: a tonsil that visibly enlarges over weeks, particularly if painless
Systemic symptoms: unexplained weight loss, drenching night sweats, persistent fevers without a clear infectious cause
Surface changes: ulceration, irregular texture, or areas of different color on the tonsil surface
Firmness or fixation: a tonsil that feels hard rather than spongy, or that seems fixed to surrounding tissue rather than freely mobile

The meta-analysis data quantify this well: without any of those features, the chance of malignancy is roughly 1 in 600. With them, particularly with lymphadenopathy, it rises to closer to 1 in 3.¹³ That enormous gap is why the presence or absence of accompanying signs matters so much more than the asymmetry itself.

What Happens When Biopsy Is Needed

If an ENT specialist decides to investigate a unilateral tonsil, the first step is often an in-clinic biopsy, sometimes combined with imaging like CT or MRI to look at the deeper tissues. The biopsy approach is not perfect. In a two-center study of 375 patients who underwent tonsil biopsies for asymmetry, about 1.6% were later found to have malignancy that was missed on the initial biopsy.¹⁶ In some of those cases, the clinic biopsy was technically difficult or the sample wasn’t representative, and a subsequent procedure under general anesthesia caught the diagnosis. Two patients had lymphoma that didn’t show up on tonsil biopsy at all and was only diagnosed through a later biopsy of neck lymph nodes.

This is worth knowing because a negative biopsy isn’t always the last word. If the clinical picture remains suspicious after a negative result, meaning the tonsil keeps growing, new symptoms develop, or something just doesn’t look right, further investigation is warranted. The decision about whether to proceed to full tonsillectomy versus monitoring depends on the balance of suspicion, the patient’s risk factors, and how technically adequate the initial biopsy was.

Vascular Causes That Mimic Tonsil Swelling

One rare but important mimic of a swollen tonsil is an aberrant internal carotid artery. The carotid arteries normally run deeper in the neck, but in some people, the vessel takes an unusual course that brings it closer to the back of the throat. This can produce a pulsating submucosal mass in the posterior pharyngeal wall that, on casual inspection, might look like a tonsillar or peritonsillar bulge.¹⁷ The danger here isn’t the artery itself but the possibility of an unwary examiner attempting a biopsy or needle aspiration and causing life-threatening bleeding. This is one reason imaging before any invasive procedure on a suspicious pharyngeal mass is considered good practice, especially when the mass pulsates or the clinical presentation is atypical.

Unilateral Swelling in Children Versus Adults

Parents tend to notice tonsil asymmetry during routine checkups or when looking in their child’s throat during an illness, and the natural reaction is worry. In most pediatric cases, one tonsil is simply more reactive than the other. Children’s tonsils are working overtime as part of the developing immune system, and temporary or even persistent size differences are common. The incidence of malignancy on pediatric tonsillectomy was only 0.13% in a large comparative study, and the single case that turned up had clinical warning signs before surgery.⁶ The researchers concluded that routine pathology of tonsillectomy specimens isn’t even necessary in children unless something in the preoperative picture raises concern.

In adults, the same finding carries more weight simply because the tonsils should have become less prominent with age. An adult tonsil that is growing rather than shrinking is inherently more suspicious than a child’s tonsil that happens to be larger on one side. The unilateral swelling is evaluated the same way regardless of age, but the urgency of investigation scales up for adults over 40 or 50 and especially for those with a smoking history, heavy alcohol use, or known HPV exposure, since these are risk factors for tonsillar squamous cell carcinoma.

When the Swelling Is Actually Next Door

Sometimes what looks like a swollen tonsil is actually swelling in the tissue surrounding it. The parapharyngeal space, a compartment that borders the tonsil on its deeper side, can harbor infections or tumors that push the tonsil inward, giving the appearance of unilateral tonsillar enlargement from the front. Deep lobe parotid tumors, parapharyngeal abscesses, and other masses in that space can all present this way. Clinicians should be suspicious when the tonsil surface itself looks normal but the overall profile bulges, or when the swelling feels deeper than the tonsil itself on palpation. Imaging is usually needed to sort this out, and the distinction matters because the surgical approach for a parapharyngeal mass is completely different from a tonsillectomy.

1
Europe PMC. Peritonsillar swelling is not always quinsy
2
BioMed Central. Tonsillar colonisation of Fusobacterium necrophorum in patients subjected to tonsillectomy
3
Hindawi / Case Reports in Otolaryngology. World’s Largest Tonsillolith Removal: A Case Report With Literature Review of Large Tonsil Stones on Record
4
PubMed. Tonsillar actinomycosis that mimics tonsillar neoplasm
5
PubMed Central. The Clinical Significance of Tonsillar Actinomyces in Histopathological Samples after Tonsillectomy
6
PubMed. Comparison of clinical and histopathological evaluation of tonsils in pediatric and adult patients
7
The Laryngoscope. A Clinicopathologic Series of 22 Cases of Tonsillar Granulomas
8
PubMed Central. Lymphangiomatous Polyp Presenting as Tonsillar Mass
9
Case Reports in Pediatrics. Unilateral Lymphangiomatous Polyp of the Palatine Tonsil in a Very Young Child: A Clinicopathologic Case Report
10
JAMA Otolaryngology–Head & Neck Surgery. A Young Man With Enlarging Unilateral Tonsil Mass
11
PubMed Central. Unilateral Tonsillar Swelling as a Manifestation of Diffuse Large B Cell Lymphoma (DLBCL): Case Report
12
PubMed. HPV-related papillary squamous cell carcinoma of the tonsil during treatment with fingolimod
13
PubMed. Tonsillar Asymmetry and Malignancy: A Meta-analysis of Diagnostic Accuracy
14
Revista Paulista de Pediatria. Tonsillar lymphoma in children with unilateral tonsillar enlargement
15
PubMed. Malignancy in asymmetrical but otherwise normal palatine tonsils
16
PubMed Central. A two-centre experience of tonsil biopsies in the investigation of patients with tonsillar asymmetry
17
PubMed Central. Imaging and clinical findings in patients with aberrant course of the cervical internal carotid arteries