An armpit lump is most often a swollen lymph node reacting to a nearby infection or minor injury. Your armpits contain 20 to 40 lymph nodes, and they swell whenever your immune system is fighting something off, even a small cut on your hand or a cold. Less commonly, armpit lumps turn out to be cysts, fatty tissue growths called lipomas, or skin conditions. In rare cases, they signal something more serious like lymphoma or breast cancer.
Swollen Lymph Nodes: The Most Common Cause
Lymph nodes are small, bean-shaped filters that trap bacteria and viruses. The ones in your armpit drain fluid from your arms, chest, and upper back, so any infection or injury in those areas can make them swell. A scrape on your finger, an infected hair follicle, or even a sore throat can trigger a noticeable lump that feels tender and slightly rubbery.
Specific infections known to cause armpit swelling include cat scratch disease (from a scratch or bite by an infected cat), which produces painful, swollen nodes that can last two to eight weeks. Mononucleosis, skin infections, and sexually transmitted infections can all cause lymph node enlargement too. In most of these cases, the lump shrinks on its own once the underlying infection clears.
Vaccine-Related Swelling
If you recently received a vaccine in your upper arm, particularly a COVID-19 or flu shot, that’s a very likely explanation. Your immune system reacts to the vaccine at the nearest lymph node, causing temporary swelling on the same side as the injection. This type of swelling can last longer than most people expect. Research published in the American Journal of Roentgenology found that vaccine-related armpit lymph node swelling took an average of 127 days (roughly four months) to fully resolve after the first dose. The Society of Breast Imaging recommends waiting at least 12 weeks before pursuing follow-up imaging for a lump suspected to be vaccine-related.
Skin Conditions That Cause Armpit Lumps
Not every armpit lump involves a lymph node. The warm, moist environment of the armpit makes it prone to several skin conditions that produce bumps.
Hidradenitis suppurativa is a chronic condition where painful, deep lumps form in areas where skin rubs together, especially the armpits and groin. In its early stages it looks like pimples or acne, which makes it easy to dismiss. The key difference is that these bumps recur in the same spots, sometimes drain pus, and can leave tunnels under the skin over time. There’s no single test for it. Diagnosis is based on the pattern of recurring lumps in characteristic locations.
Ingrown hairs and folliculitis (infected hair follicles) are also common culprits, especially after shaving. These tend to be smaller, closer to the skin surface, and often have a visible whitehead or surrounding redness. They typically resolve within a week or two with warm compresses and by avoiding further irritation.
Cysts, Lipomas, and Breast Tissue
Some armpit lumps have nothing to do with infection or inflammation. Sebaceous cysts are slow-growing, painless sacs filled with oily material that form just beneath the skin. Lipomas are soft, moveable lumps made of fat cells. Both are harmless and can sit unchanged for years.
A less well-known possibility is accessory breast tissue. Some people have extra breast tissue that extends into the armpit (separate from the main breast), and it’s frequently bilateral. Most people with accessory breast tissue never know it’s there. But it responds to hormonal changes, so it can become noticeable during pregnancy, breastfeeding, puberty, or even the premenstrual phase of your cycle, presenting as a palpable lump with pain or swelling. During initial evaluation, accessory breast tissue is commonly confused with lymph node swelling, lipomas, cysts, or even malignancy.
When an Armpit Lump May Be Serious
The characteristics of the lump itself offer important clues about whether it needs prompt attention. Lumps that are more concerning tend to be firm and hard rather than soft, fixed in place rather than moveable under your fingers, larger than about 1.5 to 2 centimeters (roughly the size of a grape or larger), and painless. Benign, infection-related lumps are usually tender, soft or rubbery, and move freely when you press on them.
Growing size and persistence matter more than any single feature. A lump that has been present for more than two weeks without an obvious cause (like a recent infection or vaccine) carries higher suspicion for something that needs investigation. Lymphoma, both Hodgkin and non-Hodgkin types, commonly presents with painless, enlarged lymph nodes. Breast, lung, skin (melanoma), and several other cancers can also spread to the axillary lymph nodes.
Breast cancer specifically deserves attention here. An armpit lump can sometimes be the first noticeable sign. Red flags that point toward a breast-related cause include a lump or thickening in the breast itself, nipple retraction on one side, unexplained skin dimpling or tethering on the breast, or bloody nipple discharge. Malignancy in accessory breast tissue, while rare (accounting for less than 1% of breast cancers), tends to have a worse prognosis largely because it’s diagnosed later due to the unexpected location.
What Happens During a Medical Evaluation
If your lump doesn’t resolve on its own within a couple of weeks, or if it has any of the concerning features above, the typical first step is a physical exam followed by an ultrasound. Ultrasound can distinguish between a fluid-filled cyst, a solid mass, and an enlarged lymph node, and it helps determine the node’s internal structure. A normal lymph node has a thin outer layer (cortex), while a thickened or irregular cortex raises suspicion.
If the ultrasound looks suspicious, the next step is usually a needle biopsy, where a small sample of tissue is drawn from the lump using a thin needle guided by ultrasound imaging. This is typically done in an office or outpatient setting and gives a definitive answer about what the lump is made of. For most people, the process from first appointment to biopsy results takes a few weeks.
Common Causes at a Glance
- Swollen lymph node from infection or injury: Tender, soft, moveable. Resolves as the infection clears, usually within two weeks.
- Vaccine reaction: Same side as injection. Can take three to four months to fully resolve.
- Ingrown hair or folliculitis: Small, near the surface, often after shaving. Clears within one to two weeks.
- Hidradenitis suppurativa: Recurring deep lumps in skin-fold areas. May drain pus and leave scarring.
- Cyst or lipoma: Painless, slow-growing, moveable under the skin. Usually harmless.
- Accessory breast tissue: May swell with hormonal changes. Often mistaken for other conditions.
- Lymphoma or metastatic cancer: Firm, fixed, painless, larger than 1.5 cm, persistent or growing.
Silicone breast implants can also trigger armpit lymph node swelling due to an inflammatory reaction if the implant leaks, so this is worth mentioning to your provider if it applies to you.