A bump under your armpit is usually a swollen lymph node, an ingrown hair, or a cyst, and most cases resolve on their own within a couple of weeks. The armpit is packed with lymph nodes, hair follicles, and sweat glands, so lumps in this area are common and rarely serious. That said, certain characteristics of the bump can tell you a lot about what’s causing it and whether it needs attention.
Swollen Lymph Nodes
The most likely explanation for a sudden armpit bump is a swollen lymph node. You have roughly 20 to 40 lymph nodes clustered in each armpit, and they swell when your immune system is fighting something off. The most common trigger is a viral infection like a cold, the flu, or mononucleosis. Bacterial infections, including strep throat, ear infections, skin wounds, and even an infected tooth, can also cause nodes in the armpit to enlarge.
A reactive lymph node (one that’s swelling in response to infection) typically feels soft or rubbery, moves freely when you press on it, and may be tender. You might also have other symptoms like a runny nose, sore throat, cough, or fever. These nodes usually shrink back to normal size once the infection clears, often within one to two weeks.
Lymph nodes under 1 centimeter (roughly the width of a pencil eraser) in their short axis are generally considered normal. Nodes larger than that, or those that keep growing, deserve a closer look.
Boils and Infected Hair Follicles
Shaving and friction make the armpit a prime spot for hair follicle problems. Folliculitis is a shallow inflammation of the follicle that shows up as small red, irritated bumps, sometimes with visible pus at the surface. It’s the mildest version and often clears up on its own with warm compresses and gentle hygiene.
A boil (also called a furuncle) is a deeper infection of the follicle that forms a painful, warm lump with a pus-filled center. It can leak whitish or bloody fluid as it drains. The bacterium Staphylococcus aureus is the most common cause of both folliculitis and boils. Warm compresses several times a day can help a boil come to a head and drain naturally. If it grows larger than a couple of centimeters, becomes extremely painful, or is accompanied by fever, it may need to be drained by a clinician.
Cysts and Lipomas
Not every armpit lump involves infection. Cysts are small, firm, fluid-filled sacs that sit just beneath the skin. They can be tender to the touch and sometimes cause redness or swelling, but many produce no symptoms at all. If a cyst ruptures, it can suddenly become painful and inflamed.
Lipomas are slow-growing fatty lumps that feel soft and doughy, move easily under the skin, and are almost never painful. They’re benign and don’t require treatment unless they bother you. The key difference: a cyst tends to feel firm and round, while a lipoma feels softer and more pliable. Both are generally small and harmless.
Reactions to Deodorant or Antiperspirant
Sometimes the bump isn’t a distinct lump at all but rather irritated, raised skin caused by a product you’re applying daily. Fragrance is the most common allergen in deodorants, present in about 90% of products. Propylene glycol, found in nearly half of formulations, is another frequent culprit. These ingredients can trigger allergic contact dermatitis, which shows up as red, bumpy, sometimes itchy patches in the armpit. In some cases, the reaction is follicular, meaning small raised bumps form around individual hair follicles and can easily be mistaken for an infection.
If your bump appeared shortly after switching products, or if both armpits are affected symmetrically, a product reaction is worth considering. Stopping the suspected product for a week or two is the simplest test.
Hidradenitis Suppurativa
If you get painful bumps in your armpit repeatedly, especially if they leave scars or seem to tunnel under the skin, you may be dealing with hidradenitis suppurativa (HS). This chronic inflammatory condition targets areas where skin rubs together, with the armpits being one of the most common sites. It typically starts in adolescence or early adulthood and tends to run in families.
The earliest sign is usually a deep, inflamed nodule that looks like a boil but doesn’t respond to typical boil treatments. Over time, HS can produce sinus tracts (tunnels under the skin that connect separate lumps), open-pored comedones, and scarring. Some people notice fatigue or tingling in the area before a flare. If your armpit bumps keep coming back in the same location, HS is worth discussing with a dermatologist, because early treatment can prevent the condition from progressing.
When an Armpit Lump Could Be Serious
In uncommon cases, an armpit lump can signal something more concerning. Lymphoma, breast cancer, and melanoma can all cause enlarged lymph nodes in the armpit. Nodes associated with malignancy tend to feel hard and fixed in place rather than soft and moveable. They grow steadily over weeks and don’t shrink on their own. Other warning signs include unexplained weight loss, drenching night sweats, and persistent fever without an obvious infection.
As a general guide, contact a healthcare provider if the lump:
- Persists beyond two weeks without shrinking
- Feels hard and doesn’t move when you push on it
- Keeps getting bigger
- Comes with fever, night sweats, or unexplained weight loss
- Returns after being removed
Most armpit bumps turn out to be something minor. A swollen node from a passing virus, an ingrown hair, or a small cyst will typically resolve with nothing more than time and basic care. Paying attention to how the lump feels, how long it lasts, and whether it’s changing gives you the clearest picture of whether it needs professional evaluation.