An armpit bump is usually one of a few things: a swollen lymph node, an infected hair follicle, a cyst, or a boil. Most are harmless and resolve on their own within a week or two. The cause often depends on what the bump feels like, how quickly it appeared, and whether it’s painful.
Swollen Lymph Nodes
Your armpits contain clusters of lymph nodes, small bean-shaped glands that filter fluid and trap infections. When your immune system is fighting something off, these nodes swell and become noticeable as firm, tender bumps under the skin. The most common cause is an infection, particularly a viral illness like the common cold, but ear infections, skin infections like cellulitis, mononucleosis, and even an abscessed tooth can trigger swelling in the armpit nodes.
A swollen lymph node from an everyday infection typically feels like a small, rubbery oval under the skin, usually less than 2 cm across. It’s often tender to the touch. Once the underlying infection clears, the node gradually shrinks back to its normal size over a few weeks. If you’ve recently had a cut, scratch, or infection on your hand, arm, or chest, that’s a likely explanation for the swelling since those areas drain directly into the armpit lymph nodes.
Infected Hair Follicles
Shaving is one of the most common reasons people develop armpit bumps. When a razor nicks a hair follicle or a hair curls back into the skin, bacteria can enter and cause folliculitis, a red, often pus-filled bump that looks like a small pimple. People who shave frequently or have curly hair are at higher risk.
Mild folliculitis usually clears up on its own within a few days if you stop shaving and keep the area clean. Switching to a trimmer instead of a razor can help prevent it from coming back. In more severe or recurring cases, the bumps can become deeper, more painful, and potentially scarring, which is worth getting looked at by a dermatologist.
Boils
A boil is a deeper, more intense infection of a hair follicle, usually caused by staph bacteria. It starts as a hard, red, painful lump and gradually fills with pus over several days, becoming softer and more swollen before it eventually drains. Boils in the armpit are common because the area is warm, moist, and subject to friction.
The standard home treatment is applying a warm, damp washcloth to the boil for about 10 minutes several times a day. This helps bring it to a head and drain on its own. Never squeeze or lance a boil yourself, as this can push the infection deeper or spread it to surrounding tissue. After touching the area, wash your hands thoroughly and launder any towels or cloths that made contact with the bump. If the boil doesn’t improve within a couple of weeks, grows larger than a golf ball, or comes with a fever, it likely needs medical drainage.
Cysts and Lipomas
Not every armpit bump is infection-related. Two common non-infectious causes are cysts and lipomas, and they feel distinctly different from each other.
A cyst feels firm, like a small balloon trapped beneath the skin. It may shift slightly when you press on it but tends to stay somewhat anchored in place. A lipoma, on the other hand, is a soft, rubbery lump made of fatty tissue. It slides easily under your fingers when you push on it. Neither is typically painful, and both are almost always benign. They can stick around for months or years without changing. Removal is usually only necessary if the bump grows, becomes uncomfortable, or bothers you cosmetically.
Hidradenitis Suppurativa
If you keep getting painful bumps in your armpit (or groin), you may be dealing with hidradenitis suppurativa, a chronic inflammatory condition that affects areas where skin rubs together. It starts with recurrent, deep lumps that can look like boils but don’t respond to typical boil treatments. In mild cases, there may be only one or a few lumps in a single area, but mild cases often progress to moderate disease over time, with more frequent flare-ups and tunnels forming under the skin.
Hidradenitis suppurativa is often misdiagnosed as recurring boils for years before someone gets the correct diagnosis. If your armpit bumps keep coming back in the same spots, that pattern itself is an important clue worth bringing to a doctor.
When the Bump Could Be Serious
Most armpit bumps are benign, but a few characteristics warrant prompt attention. A painless, hard lump that doesn’t go away after two to three weeks, especially one that keeps growing, needs evaluation. The CDC lists a new lump in the underarm as a potential warning sign of breast cancer, since cancer cells from the breast (as well as from the lungs, skin, and other organs) can spread to the axillary lymph nodes.
Lymph nodes affected by cancer tend to be round rather than oval, firm, and fixed in place rather than mobile. They also tend to be painless. By contrast, infection-related nodes are usually tender, somewhat soft, and resolve as the infection clears. A lump that persists beyond a few weeks without an obvious cause like a recent cold or skin infection is considered suspicious enough to warrant imaging, typically an ultrasound, and potentially a biopsy if the imaging looks abnormal.
What to Do First
If your armpit bump appeared suddenly and is red, warm, or tender, it’s most likely infectious: a boil, an ingrown hair, or a reactive lymph node responding to a nearby infection. Look for an obvious trigger. Did you shave recently? Do you have a cut or scratch on your arm or hand? Are you getting over a cold? These connections usually point to a straightforward cause.
For a painful, pus-filled bump, warm compresses several times a day are the simplest starting point. For a swollen lymph node during an illness, patience is usually enough. The bump that deserves the most attention is the one that doesn’t hurt, doesn’t have an obvious explanation, and doesn’t go away.