A lump in your armpit is most often a swollen lymph node reacting to an infection, and it will usually resolve on its own within a couple of weeks. Your armpit contains a dense cluster of lymph nodes, which are small, bean-shaped immune centers that filter bacteria, viruses, and abnormal cells from your upper body. When they detect a threat, they swell as immune cells multiply inside them. Less commonly, an armpit lump can be a cyst, a fatty growth, or a sign of something more serious like lymphoma or metastatic cancer.
Infections: The Most Common Cause
In the United States, routine viral and bacterial infections are by far the most frequent reason for swollen lymph nodes. A cold, the flu, or even a minor cut on your hand or arm can trigger the lymph nodes in your armpit to enlarge. The nodes swell because immune cells are rapidly multiplying inside them to fight off the invader. Macrophages inside each node remove about 99% of foreign material that passes through.
Bacterial skin infections, often caused by staph or strep bacteria, are among the most common triggers for localized swelling. Cat scratches, insect bites, or small wounds on your arm, hand, or chest wall can all lead to a swollen node on the same side. Mononucleosis and cytomegalovirus can also cause lymph node swelling, though those infections typically affect multiple node groups at once rather than just the armpit.
Vaccines Can Cause Temporary Swelling
Vaccination is a well-recognized cause of armpit lumps, particularly on the side where the shot was given. In one study of over 400 patients who received COVID-19 vaccines, about 56% of those who got an mRNA vaccine developed visible lymph node swelling in the armpit. The Moderna vaccine produced the highest rate at 79%, followed by Pfizer at 55%. This swelling is a normal immune response and not a sign of disease.
The Society of Breast Imaging recommends allowing 4 to 6 weeks after your second vaccine dose before getting a screening mammogram, since vaccine-related swelling can mimic concerning findings on imaging. If the swelling doesn’t resolve within 4 to 12 weeks, a follow-up exam is appropriate.
Cysts, Lipomas, and Other Benign Growths
Not every armpit lump involves a lymph node. Two common benign growths are lipomas and cysts, and they feel quite different from each other.
- Lipomas are clumps of fatty tissue that sit between the muscle and skin. They feel soft and doughy, and they slide easily under your finger when you press on them. They’re painless and grow very slowly.
- Cysts are fluid-filled sacs under the skin. They tend to feel firmer and are often tender to the touch. They don’t move as freely as lipomas.
Neither lipomas nor simple cysts are cancerous, though cysts can occasionally become infected and painful. Both can usually be identified by a doctor through a physical exam alone.
Hidradenitis Suppurativa
If you get recurring painful lumps in your armpit that break open, drain fluid or pus, and leave scars, you may have hidradenitis suppurativa (HS). This chronic skin condition affects areas where skin rubs together, and the armpit is one of the most common sites. Early on, it can look like acne or ordinary pimples, which makes it easy to dismiss.
Key signs that point toward HS rather than a one-time infection include blackhead-like pitting in the skin, abscesses that heal slowly and come back in the same area, tunnels that form under the skin between recurring lumps, and an unpleasant odor when the lumps drain. There’s no single test for HS. Diagnosis is based on the pattern of symptoms over time.
Silicone Implants and Armpit Lumps
If you have silicone breast implants, an armpit lump can sometimes result from an inflammatory reaction to microscopic silicone particles that leak from the implant. This can cause lymph nodes to swell even without a rupture visible on imaging. It’s not dangerous in itself, but it needs evaluation to rule out other causes.
When a Lump Could Signal Cancer
An armpit lump without any obvious infection or injury is more suspicious for a malignant cause. Cancers that can show up as an armpit lump include Hodgkin lymphoma, non-Hodgkin lymphoma, and cancers that have spread from elsewhere in the body. Breast cancer is the most well-known source of metastatic armpit nodes, but lung, thyroid, stomach, colorectal, pancreatic, ovarian, kidney, and skin cancers (especially melanoma) can also spread to the armpit.
Cancerous lumps tend to feel hard rather than soft, are often fixed in place rather than freely movable, and may or may not be painful. They typically grow steadily rather than appearing suddenly during an illness and then shrinking. Systemic symptoms like unexplained weight loss, drenching night sweats, persistent fevers, or a cough that won’t go away alongside an armpit lump raise the level of concern significantly.
How Doctors Evaluate an Armpit Lump
The first step is usually an ultrasound of the armpit. It’s quick, painless, and effective at distinguishing a normal reactive lymph node from something that needs further investigation. On ultrasound, features that raise suspicion for malignancy include a node wider than 1 centimeter on its short axis, thickened outer tissue, and the absence of a normal fatty center. That last finding, the missing fatty center, is the strongest single predictor, correctly identifying malignancy 90% to 93% of the time when present.
If the ultrasound looks suspicious, the next step is typically a needle biopsy performed under ultrasound guidance. This can be a fine needle aspiration or a core biopsy, both done in an outpatient setting. For women, a diagnostic mammogram often accompanies the ultrasound to check for an underlying breast lesion. MRI and PET scans are reserved for cases where cancer has already been confirmed and doctors need to map its extent.
Signs That Need Medical Attention
Most armpit lumps from minor infections shrink within two weeks. The Mayo Clinic recommends seeing a healthcare professional if your lump has no obvious cause, continues to grow or persists beyond two to four weeks, feels hard or rubbery, doesn’t move when you push on it, or comes with fever, night sweats, or unexplained weight loss. A lump that grows back after being removed also warrants prompt evaluation.
If you’ve recently had a cold, a cut on your arm, or a vaccination, a tender, movable swelling in your armpit is almost certainly your immune system doing its job. Give it two weeks. If it’s still there, getting bigger, or changing in character, that’s when imaging can provide a clear answer.