Ball Under Your Armpit: Causes and When to Worry

A pea-sized or marble-sized lump in the armpit is, far more often than not, a swollen lymph node reacting to a nearby infection or irritation. The armpit (axilla) houses roughly 20 to 40 lymph nodes packed into a small space, and their job is to filter bacteria, viruses, and abnormal cells from the arm, chest wall, and upper trunk. When they do that job aggressively, they swell, and you feel a “ball.” But not every armpit lump is a lymph node. Cysts, inflamed hair follicles, ectopic breast tissue, and occasionally something more serious can all present as a firm or tender mass in the same spot.

Swollen Lymph Nodes From Infection

The single most common explanation for a sudden, tender lump under the arm is reactive lymphadenopathy, meaning a lymph node has enlarged because it is fighting an infection. A cut, scratch, or infected hangnail on the hand or forearm can trigger this. So can a skin infection like cellulitis or an abscess on the chest or arm. Even a mild infection you barely notice, like a small razor nick that gets colonized by staph bacteria, can cause the nearest lymph node to balloon. Research shows that bacteria like Staphylococcus aureus travel through lymphatic fluid to the draining lymph nodes almost immediately after a skin infection begins, activating immune cells there and causing the node to swell as it mounts a defense.¹

One underappreciated trigger is cat scratch disease. If a cat scratches or bites your hand or forearm hard enough to break skin, the bacterium Bartonella henselae can enter and cause an inflamed papule at the scratch site, followed days to weeks later by swollen lymph nodes in the armpit. This pattern can produce large, firm masses that occasionally look alarming enough on imaging to be mistaken for lymphoma.² The key clue is the history: if you own a kitten or have recently been scratched by a cat, mention it to your doctor, because it changes the diagnostic picture entirely.

Viral illnesses like mononucleosis, HIV, and even a bad upper respiratory infection can also cause generalized lymph node swelling, including in the armpits. These infectious lumps are usually tender, somewhat mobile under the skin, and resolve on their own or with treatment of the underlying infection within a few weeks.

Vaccine-Related Armpit Lumps

If you recently had a vaccination in your upper arm, that lump is almost certainly a normal immune response. This gained widespread attention during COVID-19 vaccination campaigns. A systematic review found that clinically detectable armpit lymph node swelling occurred in roughly 0.4% of vaccinated people, showing up an average of about seven days after the shot, with nodes averaging around 18 mm in size. In most cases the swelling resolved within four to five weeks.³

That 0.4% figure reflects what patients could feel by touch. When researchers used sensitive PET-CT imaging to look for activated lymph nodes after the Pfizer-BioNTech vaccine, they found metabolically active nodes in over 45% of vaccinated individuals, most commonly on the same side as the injection.⁴ In other words, most people’s armpit nodes react to the vaccine; only a fraction swell enough to feel as a lump. This matters because if you happen to get an imaging scan shortly after vaccination, a lit-up armpit node could be mistaken for something worrisome if your doctor does not know about the recent shot.

While most vaccine-related lumps disappear within weeks, occasional cases persist longer. One Japanese case report documented a woman whose axillary swelling appeared seven days after her first COVID vaccine dose and, though it improved, remained detectable for six months.⁵ Persistence alone does not mean something is wrong, but a lump that lingers beyond two months after a vaccine is generally worth mentioning to your doctor so they can decide whether imaging makes sense.

Hidradenitis Suppurativa

Not every painful armpit lump is a lymph node. Hidradenitis suppurativa (HS) is a chronic skin condition that produces deep, rounded nodules in areas where skin rubs together, and the armpit is one of its favorite locations. The problem starts with a blocked hair follicle. The blockage triggers inflammation, which spreads to surrounding tissue and can form painful, pea-to-marble-sized lumps that persist for weeks to months.⁶

HS nodules tend to first appear between the ages of 20 and 30, and they recur. Early-stage HS may look and feel identical to a single boil or abscess, making it easy to dismiss as a one-off skin infection. But over time, if the condition is not managed, individual nodules can merge beneath the skin to form interconnected tunnels called sinus tracts. Drainage from these tracts may be clear, pus-like, or bloody. Eventually, repeated flare-and-heal cycles leave thick scar tissue.⁷

The hallmark that distinguishes HS from a simple infected pimple or boil is recurrence in the same area. If you keep getting painful lumps under the same arm, in the groin, or under the breasts, HS should be on the radar. Early treatment with antibiotics, anti-inflammatory medications, or biologic drugs can slow progression and reduce scarring. If you suspect HS, a dermatologist is typically the specialist best equipped to manage it.

Cysts and Lipomas

Epidermoid cysts are slow-growing, firm, round lumps that sit just under the skin. They form when skin cells that normally shed get trapped and accumulate inside a sac. Although they appear most often on the face (in one large review, about 65% of cases), they can show up anywhere, including the trunk and upper extremities.⁸ An epidermoid cyst in or near the armpit typically feels like a smooth, mobile marble that is not tender unless it gets infected. Men develop them somewhat more often than women.

Lipomas are another common culprit. These are benign fat-cell growths that feel soft, doughy, and are easily moved around under the skin. They grow slowly and are almost never dangerous, though a large one may become uncomfortable. The main practical concern with both cysts and lipomas is that, without imaging, you cannot always distinguish them from a swollen lymph node by touch alone. If a lump has been there for months, is painless, and does not change with illness, it is more likely a cyst or lipoma than a reactive node. But if you are unsure, an ultrasound can sort it out quickly.

Ectopic Breast Tissue

This is one of the less well-known causes and catches people off guard. During embryonic development, breast tissue forms along a line called the milk line that runs from the armpit to the groin. In most people, the tissue outside the normal breast area disappears before birth, but in a notable minority it persists. Ectopic (misplaced) breast tissue shows up in roughly 2 to 6% of women and 1 to 3% of men, and the armpit is by far the most common location.⁹

What makes ectopic breast tissue distinctive is that it responds to hormones the same way normal breast tissue does. Women with ectopic breast tissue in the armpit may notice swelling that comes and goes with their menstrual cycle, and it can become particularly obvious during pregnancy or breastfeeding. The tissue can also develop the same conditions as regular breasts, including fibroadenomas and, rarely, breast cancer. If you have a soft, somewhat diffuse fullness in the armpit that seems to swell cyclically, ectopic breast tissue is a possibility worth discussing with your doctor.

When to Worry About Cancer

This is the question most people are really asking, and the honest answer is that cancer causes a small minority of armpit lumps, but it is the one you do not want to miss. Lymphoma, leukemia, and metastatic cancer from elsewhere in the body (particularly breast cancer and melanoma) can all present as an enlarged axillary lymph node.

The features that raise concern include a lump that is painless and hard, feels fixed in place rather than mobile, keeps growing over weeks, or is accompanied by unexplained weight loss, drenching night sweats, or persistent fatigue. In breast cancer, axillary lymph node involvement is common. In one large study of over 1,600 breast tumors that were clinically staged as node-negative before surgery, about a third turned out to have positive axillary lymph nodes when examined under a microscope after dissection.¹⁰ That statistic is not meant to alarm you about your armpit lump; it highlights that cancer-involved nodes are not always obviously swollen, and conversely, that a swollen node is not necessarily cancer.

Research on the anatomy of axillary metastasis has shown that cancer does not always spread neatly from the lowest nodes upward. In some patients, tumors skip the first level of armpit nodes entirely and appear in deeper nodes that are harder to feel.¹¹ This is one reason why a normal-feeling armpit does not rule out cancer spreading to those nodes, and why imaging plays an important role when cancer is suspected.

How Doctors Figure Out What It Is

Your doctor will start with the basics: how long the lump has been there, whether it hurts, whether it has changed in size, and any associated symptoms like fever, recent infections, vaccinations, or unintentional weight loss. A physical exam can reveal a lot. Soft, tender, mobile lumps that appeared during or after an illness point toward reactive lymph nodes. A firm, slow-growing, painless mass without any inflammatory signs suggests a cyst, lipoma, or something that needs a closer look.

Ultrasound is the go-to first imaging tool for armpit lumps. It is fast, painless, involves no radiation, and is remarkably informative. Early research comparing ultrasound to other methods for detecting cancerous axillary nodes found that ultrasound had a sensitivity of about 73%, far better than clinical examination alone, which caught only about a third of positive cases.¹² Modern ultrasound equipment performs even better, and if a node looks suspicious on the screen, the radiologist can do a biopsy at the same visit.

When tissue sampling is needed, two main approaches are used: fine needle aspiration (FNA), which uses a thin needle to draw out cells, and core needle biopsy, which retrieves a small cylinder of tissue. In a direct comparison, core biopsy detected cancer in axillary nodes with about 88% sensitivity, while fine needle aspiration achieved roughly 73%. Both had perfect specificity, meaning a positive result was reliable either way.¹³ Core biopsy catches a few more cases, but fine needle aspiration remains a good option when the node is in a tricky spot or the patient prefers a smaller needle.¹⁴

Practical Rules of Thumb

No list of rules replaces a doctor’s evaluation, but some patterns are useful for deciding how urgently to seek care:

Tender and appeared suddenly: Most likely reactive. Think about recent cuts, infections, shaving irritation, or vaccinations. Give it two to three weeks. If it shrinks, your immune system handled whatever triggered it.
Painful, deep, and recurrent: Consider hidradenitis suppurativa, especially if you have had similar lumps in the groin or under the breasts. See a dermatologist rather than waiting for repeated flares to resolve on their own.
Painless, hard, and growing: This pattern warrants prompt medical attention. A hard, fixed, non-tender node that has been growing for more than two weeks without an obvious infectious cause should be evaluated with imaging and possibly biopsy.
Soft, mobile, unchanged for months: Likely a cyst or lipoma. Not urgent, but worth mentioning at your next checkup so your doctor can confirm.
Swelling that comes and goes with your cycle: Consider ectopic breast tissue. Mention the cyclical pattern to your doctor, as it usually narrows the diagnosis quickly.

Common Misconceptions

One persistent myth is that using antiperspirant causes armpit lumps. There is no solid evidence linking aluminum-based antiperspirants to lymph node swelling. Antiperspirant can irritate hair follicles and contribute to blocked pores, which may cause small pimples or folliculitis bumps, but these are skin-surface issues, not lymph node enlargement.

Another misconception is that a painful lump is less likely to be cancer than a painless one. While it is true that cancer-involved lymph nodes are more often painless, pain does not rule out malignancy. Some fast-growing tumors or lymphomas cause aching or pressure in affected nodes. Pain is better used as one data point among many rather than as a reason to stop paying attention.

People also commonly assume that if a lump does not go away within a couple of weeks, it must be something serious. Many benign conditions produce lumps that stick around for months. Epidermoid cysts can remain stable for years. HS nodules may wax and wane over weeks. Vaccine-related swelling occasionally persists for several months. The trend matters more than the timeline alone: a lump that is steadily growing is more concerning than one that appeared, stayed the same size, and has not changed.

Armpit Lumps in Children

Children get armpit lumps too, and the causes skew even more strongly toward the benign. Kids are constantly fighting off infections, so reactive lymph nodes are extremely common in childhood. Routine vaccinations given in the upper arm can trigger temporary armpit swelling just as in adults. Cat scratch disease is disproportionately a pediatric diagnosis, since younger children are more likely to play roughly with kittens and sustain scratches.

That said, lymphoma does occur in children, and the same red flags apply: a painless, firm, growing node without an obvious cause, especially if accompanied by unexplained fevers, weight loss, or drenching night sweats, should be evaluated. Pediatricians are generally comfortable distinguishing the common reactive node from one that needs further workup, so bringing it up at a regular visit is a reasonable first step unless the lump is growing quickly or the child seems systemically unwell.