A lump in the armpit is usually a swollen lymph node reacting to an infection, and the vast majority are harmless. Among people who visit a primary care doctor for an unexplained lump or swollen node, the rate of malignancy is estimated at just 1.1%. That said, armpit lumps have a wide range of causes, from a minor skin infection to something that needs medical attention, so understanding the differences matters.
Why Armpit Lumps Are So Common
Your armpits contain a dense cluster of lymph nodes, small filtering stations that are part of your immune system. These nodes collect fluid draining from your arm, hand, chest wall, upper back, and most of the breast tissue. That means any infection, injury, or inflammation in those areas can trigger swelling in one or more armpit nodes. A cut on your hand, a chest cold, or even a recent vaccination can cause a noticeable lump.
Most of the time, the swelling goes down on its own once the underlying trigger resolves. The most common cause of swollen lymph nodes overall is a viral infection like the common cold, strep throat, or mononucleosis. Sometimes no specific cause is ever identified, and the lump simply disappears.
Infections and Skin Conditions
Bacterial skin infections like cellulitis are a frequent reason for a tender, swollen node in the armpit. An ingrown hair, a nick from shaving, or a small wound on the arm or hand can introduce bacteria that travel to the nearest lymph nodes. Cat scratch fever, caused by bacteria transmitted through a cat scratch or bite, is another well-known cause that specifically targets the armpit nodes.
The armpit is also a prime location for hidradenitis suppurativa, a chronic condition that causes painful, pea-sized lumps under the skin in areas where skin rubs together. It typically starts with a single painful bump that lingers for weeks or months, and over time can produce recurring lumps, blackheads in small pitted areas of skin, and tunnels under the surface. This is distinct from a simple boil or infected hair follicle because it recurs and worsens without treatment.
Vaccine Reactions
If you recently received a vaccine in your upper arm, a swollen armpit node on that same side is a normal immune response. During clinical trials of the Pfizer COVID-19 vaccine, swollen armpit or neck nodes typically appeared 2 to 4 days after vaccination and lasted about 10 days. However, imaging studies found that in some people, the swelling persisted for an average of 127 days after the first dose. Because of this, radiology guidelines now recommend waiting at least 12 weeks before following up on vaccine-related swelling spotted on a mammogram, to avoid unnecessary worry or testing.
Benign Growths: Cysts and Lipomas
Not every armpit lump involves a lymph node. Two common non-node lumps are lipomas and cysts, and they feel quite different from each other.
A lipoma is a collection of fat cells between your skin and muscle. It feels soft and doughy, moves easily when you press on it, and is almost always painless. Most are small, under two inches across, and they grow very slowly if at all. They’re harmless and typically don’t need treatment unless they bother you.
A cyst, by contrast, is a small sac filled with fluid or other material. Cysts in the armpit often feel firm, can be tender to the touch, and may have redness or swelling around them. If a cyst ruptures, it can become painful. Sebaceous cysts form when the secretions from oil glands get trapped beneath the skin. Both lipomas and cysts can usually be diagnosed by feel during a physical exam, though an ultrasound can confirm the distinction if there’s any doubt.
When a Lump Could Signal Something Serious
Cancers that can cause armpit lumps include lymphoma (cancer of the lymph system), leukemia, and cancers that have spread from elsewhere, particularly breast cancer. In breast cancer, enlarged armpit nodes often indicate the cancer has reached the lymphatic system. However, physical exams alone aren’t reliable here: some people with cancer-involved nodes have no noticeable swelling, while some with clearly enlarged nodes turn out to be cancer-free.
Certain characteristics raise the level of concern. Lymph nodes larger than 1.5 centimeters in diameter (slightly bigger than a pea) are considered enlarged. Lumps that persist for more than a week or two without shrinking deserve attention. Nodes associated with lymphoma tend to be painless, moveable, and have a soft, rubbery feel. Other warning signs include unexplained weight loss, night sweats, or lumps appearing in multiple areas of the body at once.
Autoimmune conditions like lupus, rheumatoid arthritis, and sarcoidosis can also cause persistent lymph node swelling. These typically affect nodes in more than one region of the body and come with other symptoms like joint pain, skin changes, or fatigue.
How Armpit Lumps Are Evaluated
For a new, palpable armpit lump, an ultrasound of the armpit is the standard first imaging step. It allows a clear look at the size, shape, and internal structure of the lump, which helps distinguish a reactive lymph node from a cyst, lipoma, or something more concerning. In women, a diagnostic mammogram may be added to check the breast tissue for any underlying lesion, since the armpit nodes drain directly from the breast.
In one study of 171 women who had biopsies for abnormal armpit nodes found on ultrasound, only 7% turned out to be malignant. If the ultrasound reveals something suspicious, a needle biopsy guided by ultrasound can provide a definitive answer. The choice of imaging and next steps depends on age, personal risk factors, and whether the lump has features that look worrisome on the initial scan.
What to Pay Attention To
A small, tender lump that appears during or right after an illness, injury, or vaccination is almost always a normal immune response and will resolve on its own. What separates a routine swollen node from one worth investigating is persistence, size, and context. A painless lump that stays the same size or grows over several weeks, a lump harder than 1.5 centimeters, or swelling in multiple body regions at once all warrant a visit to get it checked. The same applies if you notice skin changes over the lump, if the lump feels fixed in place rather than moveable, or if it’s accompanied by symptoms like unexplained fevers or weight loss.