Most lumps under the armpit are swollen lymph nodes, and most swollen lymph nodes are caused by infections or minor injuries rather than anything serious. Your armpit contains 20 to 40 lymph nodes, making it one of the densest clusters in your body, so even a small cut on your hand or a cold can cause noticeable swelling there. That said, several other structures in the armpit can produce lumps too, and knowing the differences helps you figure out what you’re dealing with.
Swollen Lymph Nodes
Lymph nodes are small, bean-shaped filters that trap bacteria, viruses, and other invaders. When they’re fighting something off, they swell with extra immune cells and can become tender to the touch. A normal armpit lymph node measures less than 10 mm across. Once it grows beyond that, it’s considered enlarged.
The most common triggers are infections or injuries anywhere on the arm, hand, or chest on the same side. A skin infection, a cut that got a little red, or even a bad hangnail can cause an armpit node to puff up for a week or two. Upper respiratory infections, mono, and flu can cause more widespread swelling that affects both armpits along with the neck.
Some less obvious infections also target the armpit specifically. Cat scratch disease, caused by the bacterium Bartonella henselae, produces swollen, painful lymph nodes that can last two to eight weeks after a scratch or bite from an infected cat. You’ll usually notice a rash or small bumps near the wound itself before the lymph node swelling kicks in.
Vaccine-Related Swelling
Vaccinations in the upper arm frequently cause temporary lymph node swelling on that side, and this became widely recognized during COVID-19 vaccination campaigns. The swelling is a normal immune response. It resolves on its own, though it can take longer than most people expect. A study in the American Journal of Roentgenology found that the average time to resolution was about 102 days after a booster dose. Current radiology guidelines recommend waiting at least 12 weeks before pursuing imaging for armpit swelling that’s suspected to be vaccine-related, as long as there are no other concerning findings.
Skin and Sweat Gland Problems
Not every armpit lump involves a lymph node. The armpit is packed with hair follicles, sweat glands, and oil glands, all of which can become blocked or infected.
A blocked hair follicle or oil gland can form a cyst, a smooth, round, movable lump just under the skin. These are usually painless unless they become infected, at which point they turn red, warm, and tender. An abscess (a pocket of pus from a bacterial infection) can look similar but tends to be more painful and may need to be drained.
Hidradenitis suppurativa is a chronic skin condition that specifically targets areas rich in sweat glands, with the armpits being one of the most common sites. It causes recurrent, painful, inflammatory nodules under the skin that can develop into abscesses and leave scarring over time. If you keep getting painful lumps in the same area that come and go or drain fluid, this condition is worth discussing with a dermatologist. It affects roughly 1 to 4 percent of the population and is often underdiagnosed for years.
Benign Growths
Lipomas, which are soft fatty lumps, can develop in the armpit. They tend to feel doughy, move easily when you press on them, and grow slowly over months or years. They can reach several centimeters across. Lipomas are almost always harmless, though larger ones can occasionally press on nearby nerves and cause discomfort. They don’t typically need treatment unless they’re bothersome.
Silicone breast implants can also cause armpit lymph node swelling. Tiny silicone particles from implant leakage trigger an inflammatory reaction that the lymph nodes respond to, producing enlargement that can be felt as a lump.
When a Lump May Signal Cancer
A small percentage of armpit lumps are caused by cancer, either lymphoma (cancer of the lymph system) or metastatic cancer that has spread from another site, most commonly breast cancer or melanoma. Knowing the warning signs matters.
Lumps that are more concerning tend to be hard rather than soft, fixed in place rather than mobile, and painless. On ultrasound, the features radiologists watch for include a lymph node larger than 1 cm across, a thickened outer layer (greater than 3 mm), and the loss of the normal fatty center. That last finding, the absent fatty center, has the highest predictive value for malignancy at 90 to 93 percent.
Other red flags include a lump that keeps growing over weeks, swelling that appears without any obvious infection or injury, unexplained weight loss, night sweats, or a lump that persists beyond six weeks without improvement. None of these features guarantee cancer, but they do warrant prompt evaluation.
How Armpit Lumps Are Evaluated
Your doctor will start with a physical exam, checking the lump’s size, texture, and mobility, and asking about recent infections, injuries, vaccinations, or other symptoms. In many cases, especially when there’s a clear cause like a recent cold or a visible skin infection, no further testing is needed.
When imaging is warranted, ultrasound is the first-line tool for evaluating the armpit. It can assess the size, shape, and internal structure of lymph nodes and distinguish them from cysts, abscesses, or fatty lumps. The radiologist looks at whether the node has kept its normal oval shape and fatty center, or whether it has become rounded with a thickened or irregular outer layer. Mammography can visualize some armpit nodes as well, particularly those closest to the breast.
If an ultrasound reveals suspicious features, the next step is usually a needle biopsy performed under ultrasound guidance. This can be done with a fine needle or a slightly larger core needle, and it provides a tissue sample that confirms or rules out cancer. The procedure is typically quick and done with local numbing.
What to Expect With Common Causes
Most armpit lumps from infections shrink within two to four weeks as the underlying infection clears. A lump from cat scratch disease can take up to eight weeks. Vaccine-related swelling may linger for three months or more but resolves without treatment.
Cysts and abscesses sometimes resolve on their own, though abscesses often need drainage. Hidradenitis suppurativa requires ongoing management since it’s a chronic, relapsing condition. Lipomas don’t go away on their own but rarely require removal.
A new armpit lump that has no obvious explanation, that persists beyond a few weeks, or that comes with other systemic symptoms like fever, weight loss, or night sweats is worth getting checked sooner rather than later. The vast majority turn out to be benign, but the small percentage that aren’t benefit significantly from early detection.