Most armpit lumps are caused by swollen lymph nodes or minor skin conditions, not cancer. Your armpit contains roughly 20 to 40 lymph nodes, and these small, bean-shaped glands swell routinely in response to infections, injuries, and even vaccinations. That said, a lump that persists for more than two weeks, feels hard and fixed in place, or keeps growing deserves a medical evaluation.
Swollen Lymph Nodes: The Most Common Cause
Lymph nodes act as filters for your immune system, trapping bacteria, viruses, and abnormal cells. When your body fights off an infection in your arm, hand, chest, or breast, the lymph nodes in your armpit often swell as they ramp up immune activity. A cut on your finger, a skin infection on your forearm, or even a bad cold can trigger this response. The resulting lump is usually tender, somewhat soft, and moves slightly when you press on it.
Most reactive lymph node swelling resolves on its own once the underlying infection clears. If you’ve recently been sick, scratched by a cat (cat-scratch disease is a well-known cause of armpit swelling), or have any visible wound on your arm or hand, that’s likely the explanation.
Vaccines Can Cause Armpit Swelling
If you’ve recently had a vaccination in your upper arm, that’s a very common and well-documented trigger. During clinical trials for the Moderna COVID-19 vaccine, up to 16% of adults under 65 reported armpit swelling and tenderness after the second dose. Pfizer’s trials found similar effects in a smaller percentage. The swelling typically appears two to four days after the shot on the same side you were vaccinated.
What’s less widely known is how long this can last. Imaging studies found that vaccine-related lymph node swelling persists for an average of 97 days after first being detected on ultrasound, and roughly 127 days after the first vaccine dose. So if you had a vaccination a few weeks or even a couple months ago and still feel a lump, that’s within the normal range. Flu shots and other upper-arm injections can cause the same reaction, though it’s been most thoroughly studied with COVID-19 vaccines.
Skin Conditions That Create Lumps
Folliculitis and Ingrown Hairs
Shaving, waxing, and tight clothing all damage hair follicles, which can lead to inflamed, swollen bumps in the armpit. These lumps sit close to the skin surface and often look red or have a visible whitehead. Excessive sweating makes things worse by keeping the area moist and creating a breeding ground for bacteria. If you shave your armpits regularly and notice small, tender bumps that come and go, folliculitis is a likely culprit. Shaving less frequently, using a clean sharp blade, shaving in the direction of hair growth, and changing out of sweaty clothes promptly all reduce the risk.
Hidradenitis Suppurativa
This chronic skin condition causes painful, pea-sized lumps that form deep under the skin in areas where skin rubs together. The armpit is one of the most common sites. It typically starts with a single painful bump that lasts for weeks or months, then recurs. Over time, additional lumps may appear, and some break open and drain pus. In more advanced cases, tunnels form under the skin connecting the lumps, leading to scarring.
Hidradenitis suppurativa usually begins after puberty and before age 40. Women are about three times more likely to develop it than men, and Black individuals have a higher risk. If you’ve had recurring deep, painful lumps in your armpits (or groin or buttocks) that heal slowly and keep coming back, this condition is worth discussing with a dermatologist.
Cysts and Lipomas
These two benign growths feel quite different from each other. A cyst is a firm lump that may be tender to the touch and can become red or painful if it ruptures. A lipoma, by contrast, is soft and doughy, moves easily when you press it, and is almost always painless. Lipomas are typically small, around two inches or less in diameter. Neither type is cancerous, though cysts occasionally become infected and need drainage.
When a Lump Could Be Serious
The characteristics that raise concern for cancer are fairly specific. A cancerous lump in the armpit tends to be hard, painless (at least initially), and fixed in place, meaning it doesn’t move when you push on it. Breast cancer frequently spreads first to the front and center lymph nodes of the armpit, and sometimes a swollen armpit node is discovered before the breast tumor itself. Lymphomas like Hodgkin and non-Hodgkin lymphoma can also show up in the armpit, though they rarely appear there first.
When no infection, injury, or other clear trigger explains the swelling, the suspicion for something more serious goes up. Other cancers that can spread to armpit lymph nodes include lung, thyroid, skin (particularly melanoma), and several abdominal cancers. People with silicone breast implants can also develop armpit lymph node swelling from an inflammatory reaction to leaking silicone particles, which isn’t cancer but does need evaluation.
Red flags to watch for: a lump that doesn’t go away after two weeks, keeps getting bigger, feels hard and immobile, comes back after removal, or is accompanied by unexplained fever, night sweats, or weight loss.
What Happens During a Medical Evaluation
The first step is usually a physical exam. Your doctor will feel the lump’s size, texture, and mobility, and check for other swollen lymph nodes in your neck, groin, and the other armpit. For most people, the next step is an ultrasound of the armpit. This painless imaging test can determine whether the lump is solid or fluid-filled (cystic) and evaluate the structure of any enlarged lymph nodes.
On ultrasound, certain features suggest a higher chance of malignancy: a lymph node wider than 1 centimeter on its short axis, a thickened outer layer, and especially the absence of a normal fatty center. That last finding alone has a 90% to 93% accuracy rate for predicting cancer.
If the ultrasound looks suspicious, a biopsy is the next step. This is typically done with a needle guided by ultrasound. A core needle biopsy, which removes a small cylinder of tissue, catches about 88% of cancerous nodes. Fine-needle aspiration, which draws out cells with a thinner needle, catches about 74%. Both methods are highly accurate at confirming when a node is normal. The choice between the two depends on the specific situation and whether you take blood-thinning medications.
Many armpit lumps never reach the biopsy stage. If your doctor identifies a clear benign cause, like an infected hair follicle, a cyst, or a reactive node from a recent illness or vaccine, monitoring may be all that’s needed.