Why Do I Have Lumps in My Armpit? Causes & When to Worry

Most armpit lumps are swollen lymph nodes reacting to a nearby infection, a recent vaccination, or minor skin irritation, and they resolve on their own within a few weeks. The armpit (axilla) is home to roughly 20 to 30 lymph nodes packed into a small space, so even a mild immune response can produce a noticeable bump. But armpit lumps can also come from cysts, inflamed hair follicles, chronic skin conditions, and, less commonly, cancer. Understanding which features are harmless and which deserve prompt medical attention can save you both unnecessary worry and dangerous delay.

Reactive Lymph Nodes Are the Most Common Explanation

Your lymph nodes are small, bean-shaped filters scattered throughout the body, and the armpit cluster is one of the largest. When your immune system detects a threat anywhere in the arm, chest wall, or upper back, these nodes ramp up white blood cell production and swell. A cut on your hand that gets mildly infected, a bout of the flu, or even a case of strep throat can trigger a pea-sized or marble-sized lump under your arm. Doctors call this reactive lymphadenopathy, and it is overwhelmingly benign. The swelling typically peaks within a few days, feels tender to the touch, and shrinks as the underlying infection clears.

Where people sometimes get confused is when the original infection is subtle or already gone. You might not remember the small scratch on your finger, or the mild cold may have passed before the node had time to swell down. A reactive node can linger for several weeks after the triggering event, which understandably raises anxiety. As a general guide, a tender, mobile lump that appeared around the same time as an infection and is gradually shrinking is almost always a lymph node doing its job.

Vaccine-Related Armpit Lumps

If you have recently been vaccinated in your upper arm, a lump in the armpit on the same side is a well-documented immune reaction. This became a widely discussed phenomenon during the COVID-19 vaccination campaigns, but it can happen after other vaccines too, including flu shots and shingles vaccines. The mechanism is straightforward: the vaccine is injected into the deltoid, the nearby axillary lymph nodes mount a response, and the resulting swelling can be felt or even seen.

COVID-19 mRNA vaccines appear to cause this more frequently than older vaccine types. The mRNA platform triggers a particularly robust burst of immune cell activity in the lymph node’s germinal center, which makes swelling more likely compared to protein-based vaccines.¹ In terms of timing, swelling has been reported starting as early as a day or two after vaccination, but in some people it appears about a week later and can persist for weeks or even months. A Japanese review found that axillary lymph node swelling occurred around seven days after vaccination and in some cases remained detectable for up to six months.²

The practical issue here is that a vaccine-related node can look suspicious on a mammogram or ultrasound, potentially triggering an unnecessary cancer scare. If you are scheduled for breast imaging shortly after a vaccination, mention the vaccine to your radiologist. Some imaging guidelines suggest waiting four to six weeks after vaccination if you are due for a routine mammogram, though you should not delay screening that was ordered because of a clinical concern.

Cat Scratch Disease and Other Zoonotic Infections

A surprisingly common and frequently overlooked cause of armpit lumps is cat scratch disease, an infection caused by the bacterium Bartonella henselae. If a cat scratches or bites your hand or arm hard enough to break the skin, bacteria can travel through the lymphatic system to the axillary nodes, causing them to swell dramatically. The nodes can grow large enough to be alarming, sometimes mimicking the appearance of a tumor on imaging.

One case report describes a 24-year-old man who developed painful, progressively enlarging masses in his upper arm and armpit over four months after sleeping regularly with his cat and sustaining a bite. Ultrasound and biopsy ultimately confirmed cat scratch disease rather than the lymphoma initially suspected.³ In another case, a 76-year-old man with a compromised immune system developed isolated axillary swelling after a cat bite to the finger on the same side. He was treated with oral antibiotics and the infection eventually drained on its own.⁴

What makes cat scratch disease tricky is that it does not always come with the textbook package of fever and malaise. It can present as nothing more than swollen axillary lymph nodes without other symptoms, which means the differential diagnosis is wide and includes active infection, inflammatory processes, and metastatic disease.⁵ If you have cats at home and develop a new armpit lump, mention the pets to your doctor. It could save you from an unnecessary biopsy.

Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a chronic skin condition that produces painful, deep-seated nodules and abscesses in areas where skin rubs together, and the armpit is one of its favorite locations. Unlike a one-off swollen lymph node, HS tends to recur in the same spots over months or years. The lumps can rupture, drain foul-smelling fluid, and eventually form scar tissue and tunnels (sinus tracts) under the skin.⁶

The underlying problem involves inflammation and destruction of hair follicles in areas rich in apocrine sweat glands, with immune dysregulation playing a central role.⁷ HS is often mistaken for “just boils” or recurring ingrown hairs, and many people live with it for years before getting a proper diagnosis. The pattern to watch for is repeated painful lumps in the armpits (or groin, under the breasts, or between the buttocks) that come back in roughly the same areas, sometimes leaving tracks of scarring between episodes.

If this description sounds familiar, a dermatologist can assess the severity and discuss treatment options, which range from topical antibiotics for mild cases to biologic medications for severe or widespread disease. HS is not caused by poor hygiene, though the stigma around the condition leads many people to suffer quietly.

Cysts, Lipomas, and Other Benign Growths

Not every armpit lump involves a lymph node or an inflamed follicle. Benign soft tissue growths are common throughout the body, and the armpit is no exception. The two you are most likely to encounter are epidermoid cysts and lipomas.

Epidermoid cysts: These develop when skin cells get trapped beneath the surface and form a sac that slowly fills with keratin. They feel like firm, round bumps and tend to stay the same size for a long time, though they can occasionally become inflamed or infected.
Lipomas: These are soft, rubbery lumps made of fat cells sitting just under the skin. They move easily when you press on them, grow slowly, and are almost never dangerous. They can appear anywhere fat tissue exists, and the armpit has plenty of it.

On imaging, these growths have distinct characteristics. Ultrasound studies have shown that epidermoid cysts are noticeably stiffer than lipomas, which helps doctors tell them apart without surgery.⁸ The practical difference for you is that a lipoma typically feels soft and squishy, while an epidermoid cyst feels firmer and more defined. Neither one requires treatment unless it is causing pain, growing rapidly, or bothering you cosmetically.

Accessory Breast Tissue

This one surprises most people: a lump in the armpit can actually be breast tissue. During embryonic development, a ridge of tissue called the milk line runs from the armpit down to the groin on each side. Normally, only the tissue at the chest develops into breasts, but occasionally small islands of breast tissue persist elsewhere along that line, with the armpit being the most common extra location.

Accessory breast tissue often goes unnoticed for years because it is small and soft. It tends to make itself known during hormonal surges, particularly pregnancy and breastfeeding, when it swells and can even produce milk.⁹ One case report describes a 28-year-old woman who developed bilateral armpit masses during her first pregnancy that grew progressively in size. During breastfeeding, a milk-like fluid drained through small rudimentary nipples in the center of each mass, and MRI confirmed the tissue was breast tissue, separate from her normal breasts but behaving identically.¹⁰

Accessory breast tissue is benign, but because it is real breast tissue, it carries the same small risk of developing the conditions that affect normal breasts, including cysts, fibroadenomas, and very rarely, breast cancer. If you have a persistent soft lump in the armpit that swells with your menstrual cycle or during pregnancy, your doctor may want to confirm whether it is accessory breast tissue so it can be monitored appropriately.

Autoimmune and Rare Inflammatory Conditions

Swollen axillary lymph nodes are sometimes the first visible sign of a systemic autoimmune disease. Lupus (systemic lupus erythematosus, or SLE) is one of the more recognized culprits, but the connection is not always obvious because the lymph node swelling may appear before the classic rashes, joint pain, or fatigue that point toward lupus.

In one reported case, a 16-year-old girl initially presented with axillary lymphadenopathy and was diagnosed with Kikuchi-Fujimoto disease, a rare, self-limiting condition that causes painful lymph node swelling. She later met the full diagnostic criteria for SLE.¹¹ Kikuchi-Fujimoto disease itself can cause isolated axillary lumps. In another case, a 56-year-old man had a node removed from his armpit and was diagnosed with Kikuchi disease on biopsy, without any other symptoms at all.¹²

These cases are uncommon, but they illustrate an important point: a persistent, unexplained armpit lump sometimes has a systemic cause. If your doctor cannot find an obvious local infection or skin condition to explain the swelling, blood work and biopsy can help uncover less common diagnoses.

When an Armpit Lump Could Signal Cancer

This is the worry most people carry into a search engine, so it deserves a direct answer. Yes, armpit lumps can be a sign of cancer, but they are much more likely to be benign. The cancers most associated with axillary lymph node swelling are breast cancer (when tumor cells spread through the lymphatic system to the nearest nodes) and lymphoma (a cancer of the lymphatic system itself).

A cancerous lymph node tends to behave differently from a reactive one. Warning signs include a lump that is hard and fixed rather than soft and mobile, one that continues to grow over several weeks rather than shrinking, a painless lump (reactive nodes are usually tender), and lumps accompanied by unexplained weight loss, drenching night sweats, or persistent fevers. No single feature is diagnostic on its own, but a combination of these raises the level of concern.

Research into breast cancer metastasis to axillary nodes shows that cancer cells do not spread uniformly through all the armpit’s lymph nodes at once. Instead, individual nodes respond independently to tumor cells arriving from the breast, meaning early-stage spread may involve only one or two nodes while the rest remain normal.¹³ This is part of why doctors pay close attention to the specific characteristics of individual nodes during imaging and examination, rather than treating the armpit as one undifferentiated mass of tissue.

For lymphoma, the lump tends to be rubbery rather than rock-hard, and it can grow fairly quickly. Lymphoma nodes are typically painless and may appear in multiple node groups at once: armpits, neck, and groin. If you have swelling in more than one area without an obvious infection driving it, your doctor will want to evaluate promptly.

How Doctors Evaluate an Armpit Lump

If your armpit lump does not resolve on its own within a couple of weeks, or if it has concerning features from the start, your doctor will likely begin with a physical exam and history. They will want to know when you first noticed the lump, whether it is growing, whether it hurts, and whether you have had any recent infections, vaccinations, scratches, or other symptoms.

Ultrasound is typically the first imaging tool used for armpit lumps. It can distinguish between solid masses and fluid-filled cysts, measure the thickness of the lymph node cortex (the outer layer that thickens when something is wrong), and detect abnormal shape or blood flow patterns. One large multi-site study found that routinely scanning the armpit during breast ultrasound yields a low rate of incidental findings and rarely changes cancer detection, which suggests that most armpit abnormalities that show up on imaging are benign.¹⁴

If the ultrasound shows something that needs further evaluation, the next step is usually a biopsy. Two main approaches exist: fine needle aspiration (FNA), which uses a thin needle to draw out cells, and core needle biopsy (CNB), which uses a slightly larger needle to extract a small cylinder of tissue. A meta-analysis comparing the two in the context of suspected lymph node metastasis found that core biopsy was more sensitive, picking up about 88% of cancerous nodes compared to 74% for fine needle aspiration. Both had perfect specificity, meaning a positive result was reliable either way.¹⁵ The trade-off is that core biopsy carries a somewhat higher complication rate, while fine needle aspiration is more likely to produce an inconclusive sample that requires a repeat procedure.¹⁵

In practice, the choice between the two depends on the specific situation. If the suspicious node is easy to reach and clearly visible on ultrasound, a core biopsy gets more definitive tissue. If the node is in a tricky location or the patient prefers a smaller needle, fine needle aspiration is a reasonable first step.¹⁶ Either way, the goal is the same: get a sample of the tissue and examine it under a microscope to determine what is actually going on.

Skin Irritation and Contact Reactions

Sometimes the lump in your armpit is not deep tissue at all. Superficial irritation from shaving, waxing, or allergic reactions to deodorants and antiperspirants can produce inflamed bumps that feel like lumps. An ingrown hair after shaving can swell into a firm, painful nodule that looks alarming but is really just trapped hair and surrounding inflammation. These usually improve on their own with warm compresses and by leaving the area alone for a few days.

Allergic contact dermatitis from fragrances or preservatives in underarm products can also cause a diffuse, bumpy rash that might feel lumpy when you run your hand over it. Switching to a fragrance-free product and giving the skin time to recover typically resolves the issue. If you keep getting lumps in the same area every time you switch back to a particular product, the product is likely the problem. A patch test from a dermatologist can confirm the specific ingredient you are reacting to.

Practical Guidelines for Self-Assessment

Given the range of possible causes, a few rules of thumb can help you gauge urgency. Consider seeking medical evaluation sooner rather than later if any of these apply:

The lump persists beyond two to three weeks without any shrinkage, especially if you cannot identify a recent infection or vaccination that might explain it.
It is hard, fixed, and painless. Benign reactive nodes are typically soft or rubbery, move freely under the skin, and hurt when you press them. A lump that does not move and does not hurt is more concerning.
It is growing steadily. A lump that is larger this week than last week deserves attention regardless of how it feels.
You have systemic symptoms. Unexplained weight loss, drenching night sweats, persistent fevers, or profound fatigue alongside an armpit lump all warrant evaluation.
You have a personal or strong family history of breast cancer or lymphoma. The threshold for imaging should be lower if your baseline risk is elevated.

On the other hand, a small, tender, mobile lump that appeared after a vaccination, a cold, or a minor skin wound is very likely to be a reactive lymph node. Giving it two to three weeks to resolve before seeking imaging is generally reasonable, provided you are otherwise feeling well and the lump is not rapidly enlarging. Many people discover armpit lumps during moments of heightened body awareness, like during a breast self-exam, and the vast majority turn out to be nothing serious. The key is knowing which features should shorten your wait to see a doctor rather than letting anxiety or avoidance make that decision for you.