Most armpit lumps are swollen lymph nodes reacting to a nearby infection, and they resolve on their own within a couple of weeks. Your armpit contains 20 to 40 lymph nodes, and these small, bean-shaped filters swell whenever they trap bacteria, viruses, or other foreign material. But infections aren’t the only explanation. Cysts, fatty growths, skin conditions, vaccine reactions, and occasionally cancer can all produce a noticeable lump in this area.
Swollen Lymph Nodes: The Most Common Cause
A healthy armpit lymph node measures less than 10 millimeters across, roughly the size of a pea. When a node swells beyond that, you can usually feel it by pressing into the hollow of your armpit. The most frequent trigger is a localized infection: a cut on your hand, a skin infection on your arm, or an infected hair follicle nearby. Bacteria like Staphylococcus and Streptococcus are the usual culprits. The node traps those bacteria, mounts an immune response, and puffs up in the process.
Body-wide infections cause swelling too. Mononucleosis, flu, and other viral illnesses can enlarge lymph nodes in multiple areas at once, including the armpits. Less common infections like tuberculosis and cat scratch disease (from a scratch or bite by an infected cat) also target these nodes specifically. In most of these cases, the lump is tender, somewhat soft, and moves slightly when you press on it. Once the infection clears, the swelling gradually fades.
Cysts and Abscesses
A cyst is a small sac filled with fluid or semi-solid material that forms under the skin. In the armpit, cysts often develop from blocked sweat glands or hair follicles, sometimes triggered by shaving or antiperspirant use. They feel firm, like a small balloon sitting just beneath the surface, and they may be slightly anchored in place rather than sliding freely. An uninfected cyst is usually painless, but if bacteria get inside, it can turn into an abscess: red, swollen, warm, and painful, sometimes draining pus.
Lipomas
Lipomas are harmless fatty growths that can appear almost anywhere under the skin, including the armpit. They feel distinctly different from other lumps. A lipoma is soft, rubbery, and doughy, and it slides easily under your fingertip when you press on it. They’re almost always painless and grow very slowly over months or years. Lipomas don’t become cancerous. Most people leave them alone unless the lump becomes large enough to be bothersome, in which case a doctor can remove it with a simple procedure.
Vaccine Reactions
If you recently received a vaccination in your upper arm, particularly a COVID-19 vaccine, a swollen lymph node on that same side is a well-documented response. Your immune system is doing exactly what the vaccine asked it to do: recognizing foreign material and building a defense. After a COVID-19 booster dose, armpit swelling typically improves within about six weeks, though in some cases imaging has shown lingering enlargement averaging around 100 days after injection. After the initial two-dose series, swelling can occasionally persist even longer, with one study detecting it on imaging up to 43 weeks later. Flu shots and other vaccines can produce the same reaction, usually resolving faster.
Hidradenitis Suppurativa
If you keep getting painful lumps in your armpit that heal slowly and come back, you may be dealing with a chronic skin condition called hidradenitis suppurativa. It typically starts as a single, painful, pea-sized bump under the skin that lingers for weeks or months. Over time, more bumps appear in areas where skin rubs together: armpits, groin, buttocks, and under the breasts.
This is not the same as a simple boil or ingrown hair. The bumps fill with pus, may break open and drain fluid with a noticeable odor, and can eventually form tunnels beneath the skin that connect the lumps. Paired blackheads in small pitted areas of skin are another hallmark. Hidradenitis suppurativa doesn’t go away on its own and tends to worsen without treatment, so recognizing the pattern early matters. A dermatologist can help manage flare-ups and slow progression.
When a Lump Could Be Cancer
Cancer is a less common but important cause of armpit lumps. Lymphomas (cancers of the lymphatic system) and breast cancer that has spread to nearby nodes are the primary concerns. A cancerous lymph node tends to feel rock-hard and fixed in place, shaped more like a marble than the flat, oval shape of a normal node. On ultrasound, suspicious nodes show thickened outer tissue measuring more than 3 millimeters, with focal thickening above 6 millimeters considered highly suspicious.
One counterintuitive detail: a cancerous lump in the armpit is more likely to be painless. Painful lumps are more often tied to infection or inflammation. That said, pain alone doesn’t rule anything in or out. The combination of a hard, immovable, irregularly shaped lump that doesn’t resolve deserves prompt evaluation.
What to Track While You Wait
Not every armpit lump needs an immediate doctor’s visit. If you’ve recently been sick, cut yourself shaving, or gotten a vaccine, a tender, movable lump is likely your immune system at work. Give it about two weeks and pay attention to whether it’s shrinking.
Contact your doctor if the lump:
- Persists beyond two weeks without getting smaller
- Feels hard and doesn’t move when you press it
- Keeps growing rather than staying the same size or shrinking
- Comes with a fever or other signs of spreading infection, like red streaks on the skin
- Becomes newly tender after initially being painless
- Returns after previously being removed
How Doctors Evaluate an Armpit Lump
A physical exam is usually the first step. Your doctor will feel the lump’s size, shape, texture, and mobility, and ask about recent illnesses, injuries, or vaccinations. If the cause isn’t obvious from context, the next step is typically an ultrasound, which can distinguish between a fluid-filled cyst, a solid mass, and an enlarged lymph node while also measuring the node’s internal structure.
If imaging raises concerns, a needle biopsy may follow. This involves inserting a thin needle through the skin into the lump to extract a small tissue sample for lab analysis. It’s a quick outpatient procedure and can confirm or rule out cancer, unusual infections, or immune conditions like sarcoidosis. In cases where cancer is already diagnosed elsewhere in the body, a sentinel node biopsy (removing the lymph nodes closest to the tumor) helps determine whether the cancer has spread.