Most armpit lumps are caused by swollen lymph nodes or minor skin irritations, not something dangerous. Your armpit contains 20 to 40 lymph nodes, and they swell routinely in response to infections, skin injuries, and even recent vaccinations. That said, the feel, size, and behavior of the lump matter, and some combinations of features do warrant a closer look.
Swollen Lymph Nodes: The Most Common Cause
Your lymph nodes are small, bean-shaped filters that trap bacteria, viruses, and other unwanted material. When your immune system is fighting something off, nearby nodes can swell to several times their normal size. In the armpit, this commonly happens in response to a cut or infection on your hand, arm, or chest wall. A cold, the flu, or mononucleosis can also trigger it. Even a recent vaccination in your upper arm can cause noticeable swelling that lasts a few days to a couple of weeks.
Reactive lymph nodes from infections tend to be soft, movable, and sometimes tender to the touch. They often appear on both sides of the body and have clear borders you can feel with your fingertips. In most cases, they shrink back to normal within two to four weeks as the underlying infection clears.
Skin Irritations and Infected Hair Follicles
Shaving, tight clothing, and certain deodorants can irritate the delicate skin in your armpit and create small, painful bumps. Folliculitis, an infection of the hair follicle, produces clusters of pimple-like bumps that may be itchy, tender, or filled with pus. These are superficial and typically resolve on their own or with warm compresses.
Deeper infections of the follicle can form boils: red, painful lumps that grow over a few days as pus collects beneath the skin. A boil tends to appear suddenly and may feel warm to the touch. Carbuncles, which are clusters of connected boils, create a larger area of infection and can be accompanied by fever. These deeper infections sometimes need drainage but are still benign.
Hidradenitis Suppurativa
If you’ve had recurring, painful nodules or boils in your armpits (or groin) over the past six months or longer, the cause may be a chronic skin condition called hidradenitis suppurativa. It starts with inflamed nodules and abscesses, often alongside blackheads and whiteheads. Over time, repeated flare-ups can create narrow channels under the skin that drain fluid, along with significant scarring. The condition tends to progress through stages: early on, abscesses may heal without scarring, but in more advanced cases, connected tracts and thick scar tissue develop with little normal skin remaining between them.
Hidradenitis suppurativa is frequently misdiagnosed as ordinary boils. A useful screening: if you’ve had repeated outbreaks of big, painful nodules in the armpits or groin that heal with scars, there’s roughly a 90 percent chance this is the diagnosis. It’s more common in people who also have severe acne and tends to begin in the teens or twenties.
Lipomas and Cysts
Not every armpit lump involves lymph nodes or infection. Lipomas are slow-growing fatty lumps that sit just beneath the skin. They feel soft and doughy, move easily when you press on them, and are usually painless. Most stay under two inches in diameter. They’re harmless and don’t turn into cancer, though they can be removed if they bother you.
Cysts, by contrast, tend to feel firmer. They may be tender to the touch, and the surrounding skin can become red or swollen. If a cyst ruptures, it can become painful and inflamed. Both lipomas and cysts are benign growths, but a cyst that becomes infected may need to be drained.
Cat Scratch Disease and Other Infections
A swollen armpit node that appears three to ten days after a cat scratch or bite could be cat scratch disease, a bacterial infection spread through broken skin. The most recognizable pattern is a small rash or bumps near the wound followed by a swollen, painful lymph node in the armpit on the same side. You might also develop a low fever, fatigue, and muscle aches. The swollen node can persist for two to eight weeks. Most cases resolve without treatment, though antibiotics are sometimes used for severe symptoms.
Other infections that specifically cause armpit lymph node swelling include skin infections from staph or strep bacteria (common after cuts or abrasions on the hand or arm), Lyme disease, and tuberculosis, though these are less typical in everyday settings.
Breast Tissue in the Armpit
Breast tissue doesn’t stop neatly at your chest wall. A narrow extension called the axillary tail reaches into the armpit area. Both normal breast tissue and lymph nodes sit in this region, which means lumps here can sometimes be breast-related. Hormonal changes during your menstrual cycle, pregnancy, or breastfeeding can cause this tissue to swell temporarily.
In rare cases, a lump in this area can represent breast cancer that has either arisen in the axillary tail itself or spread to nearby lymph nodes. This is one reason why clinical breast exams include the armpit.
Features That Distinguish Harmless From Concerning Lumps
The physical characteristics of your lump tell a meaningful story. Nodes that are hard, painless, and fixed in place (meaning they don’t slide around when you press them) carry more significance for serious conditions like lymphoma or metastatic cancer. Lymphoma nodes tend to feel firm and rubbery with well-defined edges. They don’t hurt, which can be counterintuitive since people often assume pain equals danger.
Tender or painful lumps, on the other hand, usually signal an infection or inflammatory reaction. While pain isn’t a perfect rule (rarely, a rapidly growing tumor can be painful), it’s generally a reassuring sign.
Size matters too, but growth over time matters more. A node that keeps getting bigger over weeks, rather than shrinking, is more concerning than a large node that appeared suddenly during an infection and is already getting smaller. Nodes larger than about 1.5 to 2 centimeters (roughly the size of a grape or larger) tend to prompt further evaluation, especially when there’s no obvious infection to explain them.
Systemic symptoms alongside a persistent lump raise the concern level further. Unexplained weight loss, drenching night sweats, fevers without a clear source, and a cough that won’t resolve are all red flags when they accompany an armpit lump that has been present for more than two to four weeks.
How Armpit Lumps Are Evaluated
When a lump needs further workup, ultrasound is the standard first step. The American College of Radiology recommends ultrasound of the axilla as the initial imaging study for a new palpable armpit lump, whether it’s on one side or both. Ultrasound is quick, painless, and effective at distinguishing between solid masses, fluid-filled cysts, and enlarged lymph nodes. It can also reveal features that suggest whether a node looks reactive (likely benign) or suspicious.
If ultrasound identifies a suspicious node, the next step is usually an ultrasound-guided biopsy, either through fine-needle aspiration or a core needle biopsy. Fine-needle aspiration is a reliable, low-complication procedure. In studies, adding needle biopsy to ultrasound improved detection sensitivity from about 61 percent to 79 percent, with specificity reaching 100 percent. That means when the biopsy says it’s not cancer, it’s almost certainly not cancer.
Mammography isn’t typically the first-line test for an armpit lump on its own, but it may be added if there’s a personal history of breast cancer or clinical suspicion of a breast-related cause. MRI and PET scans are reserved for specific scenarios later in the diagnostic process, not for initial evaluation.
Medications That Cause Swollen Nodes
Some commonly prescribed medications can trigger lymph node swelling as a side effect. These include certain blood pressure medications, seizure medications, antibiotics, and gout treatments. If you started a new medication in the weeks before noticing the lump, it’s worth mentioning to your provider. Drug-related lymph node swelling typically resolves after stopping the medication.
Autoimmune Conditions
Chronic lymph node swelling in the armpit can occasionally be a feature of autoimmune diseases like lupus, rheumatoid arthritis, or Sjögren syndrome. In these cases, the swollen nodes are rarely the only symptom. You’d typically also have joint pain, fatigue, rashes, dry eyes, or other signs that something systemic is going on. If you have a known autoimmune condition and develop new armpit swelling, it may reflect a disease flare rather than a new problem.