Most armpit lumps are swollen lymph nodes reacting to a nearby infection, a recent vaccine, or minor skin irritation. The armpit contains 20 to 40 lymph nodes, and they swell routinely as part of your immune response. Less commonly, an armpit lump turns out to be a cyst, a fatty growth, or a skin condition. Rarely, it signals something more serious like lymphoma or breast cancer.
Swollen Lymph Nodes
The most common reason for an armpit lump is a swollen lymph node. Your armpit nodes filter fluid from your arm, chest wall, lateral breast, and lower neck. When your body fights off an infection anywhere in those areas, the nodes can swell to the size of a marble or larger. A cut on your hand, a cold, strep throat, or even a mild skin infection from shaving can trigger it. Swollen nodes from infection tend to feel soft, tender, and movable under the skin.
Cat scratch disease is a specific infection worth knowing about. If a cat scratches or bites your hand or arm, bacteria called Bartonella henselae can travel to the nearest lymph nodes and cause noticeable swelling. These swollen nodes typically last two to eight weeks and resolve on their own without treatment.
Vaccine-Related Swelling
If you recently got a vaccine in your upper arm, that’s a likely explanation. COVID-19 vaccines, flu shots, and other injections can cause the lymph nodes on the same side to swell as your immune system mounts a response. In one study of over 200 patients who received COVID-19 vaccines, 36% still had reactive lymph nodes visible on imaging up to 10 weeks after vaccination. The swelling was more common in women and in people under 65. It gradually decreases in both size and intensity over time and is completely harmless.
Cysts and Lipomas
Not every armpit lump involves lymph nodes. Two common benign growths can appear in the armpit: cysts and lipomas.
A cyst is a firm lump that may feel tender to the touch, with redness or swelling around it. Cysts can become painful if they rupture or get infected. A lipoma, by contrast, feels soft and doughy, moves easily when you press on it, and is usually painless. Lipomas are typically under two inches in diameter. Neither type is cancerous, though a cyst that becomes red, hot, or painful may need draining.
Hidradenitis Suppurativa
If you keep getting painful lumps in your armpit that come and go, you may be dealing with hidradenitis suppurativa. This chronic skin condition usually starts with a single painful bump under the skin that persists for weeks or months. Over time, more bumps form in areas where skin rubs together or where sweat glands are concentrated: armpits, groin, buttocks, and under the breasts.
The lumps can break open and drain pus with an odor. They heal slowly and often recur, eventually creating tunnels under the skin and leaving ropelike or pitted scars. Early signs include small paired blackheads and pea-sized painful lumps. In advanced cases, scar tissue in the armpit can restrict arm movement. If this pattern sounds familiar, it’s worth getting evaluated, because earlier treatment can prevent scarring and progression.
Skin Irritation From Grooming Products
Sometimes what feels like a lump is actually inflamed, bumpy skin from contact dermatitis. If your armpit is itchy, red, flaking, or bumpy, your deodorant or antiperspirant may be the cause. Irritant contact dermatitis happens when a product directly damages the skin. Allergic contact dermatitis is a delayed reaction that appears as a scaly, itchy rash a day or two after exposure to a specific ingredient. Switching to a fragrance-free or aluminum-free product for a few weeks can help you determine whether your product is the trigger.
Normal Breast Tissue in the Armpit
Breast tissue naturally extends into the armpit in an area called the tail of Spence. This tissue connects to the breast’s normal duct system and is present in everyone with breasts. It can become more noticeable during pregnancy, breastfeeding, hormonal changes, or weight fluctuations. When engorged, it may feel like a distinct lump or cause localized pain. If the “lump” seems to change with your menstrual cycle or feels like the same texture as the rest of your breast, this is a likely explanation.
When an Armpit Lump May Be Serious
The vast majority of armpit lumps are benign, but certain characteristics raise concern. Lumps that feel hard, fixed in place (not movable), and painless are more suspicious for cancer than soft, tender, movable ones. Firm, rubbery nodes can suggest lymphoma, while rock-hard, immovable nodes are more typical of metastatic solid tumors. Risk factors for a malignant cause include being over 40, having a lump larger than two centimeters, unexplained weight loss, night sweats, or persistent fevers.
You should have an armpit lump evaluated if it doesn’t go away after two weeks, feels hard and painful, keeps growing, grows back after being removed, comes with a fever or signs of infection, or suddenly becomes tender when it wasn’t before.
What Happens at a Doctor’s Visit
Your doctor will start with a physical exam, checking the lump’s size, texture, and mobility. If the lump feels suspicious or doesn’t resolve, the next step is usually an ultrasound. On imaging, features that raise concern include a lymph node wider than one centimeter on its short axis, thickened outer tissue, and the absence of the normal fatty center that healthy lymph nodes have. That last finding, a missing fatty center, is the strongest predictor of malignancy on ultrasound, with a 90% to 93% accuracy rate.
If the ultrasound looks concerning, a biopsy follows. This is typically done with a needle guided by ultrasound, either a fine-needle aspiration or a core needle biopsy. Core biopsies are generally preferred because they capture more tissue and have higher accuracy (around 88% sensitivity compared to 74% for fine-needle aspiration). The specificity of both approaches is excellent, ranging from 98% to 100%, meaning a negative result is highly reliable.