Most armpit lumps are caused by swollen lymph nodes reacting to a nearby infection, or by common skin issues like ingrown hairs and cysts. In the vast majority of cases, especially in younger adults, the cause is benign. That said, the characteristics of the lump matter. Understanding what different types of lumps feel like and when they need medical attention can save you both unnecessary worry and missed warning signs.
Swollen Lymph Nodes: The Most Common Cause
Your armpits contain a dense cluster of lymph nodes, small bean-shaped structures that filter fluid and trap bacteria, viruses, and other threats. When your immune system is fighting something off, these nodes swell as they ramp up production of infection-fighting cells. This is the single most frequent reason people notice a lump in the armpit.
The infections behind swollen armpit lymph nodes range from minor to serious. Common culprits include skin or wound infections (cellulitis), strep throat, ear infections, mononucleosis, and even an abscessed tooth. Less common infections like cat scratch fever, tuberculosis, and certain sexually transmitted infections can also trigger armpit swelling. Autoimmune conditions like lupus and rheumatoid arthritis sometimes cause it too, since the immune system is chronically activated.
A swollen lymph node from infection typically feels soft or slightly firm, is tender to the touch, and moves freely under the skin when you press it. It usually shrinks back to normal within two to four weeks once the underlying infection clears.
Skin Irritation and Ingrown Hairs
Shaving, waxing, or tweezing removes hair but leaves the follicle behind. Sometimes the hair grows back and curls under the skin instead of growing straight out, clogging the follicle. Skin cells and a protein called keratin collect in the blocked space, forming a cyst that can feel like a firm, painful bump right at the skin’s surface. These ingrown hair cysts are extremely common in the armpit because of how frequently the area is shaved.
Folliculitis, an infection at the root of a hair strand, creates a similar picture: a red, tender bump that may have a visible whitehead. Tight clothing, heavy sweating, and dull razors all raise the risk. These lumps are usually small, close to the surface, and resolve on their own or with warm compresses within a week or so.
Lipomas and Other Benign Growths
Lipomas are round or oval lumps made entirely of fat that grow just beneath the skin. They feel rubbery (not hard), move easily when you touch them, and are almost always painless. They grow slowly and don’t spread to surrounding tissue. Most are smaller than two inches across, though occasionally they get larger. Lipomas appear most often on the back, shoulders, arms, and neck, but the armpit is another common spot.
Because lipomas and cysts can feel similar to each other and to other types of lumps, imaging like ultrasound is sometimes used to tell them apart. Neither lipomas nor simple cysts are cancerous, and they generally don’t require treatment unless they’re bothering you.
Vaccine-Related Swelling
If you’ve recently been vaccinated in your upper arm, a lump in the armpit on that same side is a well-documented immune response. This was widely recognized during COVID-19 vaccination, but it can happen with flu shots and other vaccines too. The lymph nodes swell because they’re doing exactly what they’re supposed to: mounting an immune response to the vaccine.
Research tracking patients with vaccine-related armpit swelling after COVID-19 boosters found it took an average of about 102 days to fully resolve on ultrasound. After a first dose in the initial vaccine series, resolution took even longer, averaging 129 days. Current guidelines recommend waiting at least 12 weeks before pursuing follow-up imaging for suspected vaccine-related lymph node swelling, since it takes that long to clear in many people.
Breast Tissue in the Armpit
Something many people don’t realize is that breast tissue extends into the armpit. This extension, called the axillary tail, is a normal part of breast anatomy, not extra or misplaced tissue. It contains both breast tissue and lymph nodes. Hormonal changes during menstrual cycles, pregnancy, or breastfeeding can cause this tissue to swell, creating a lump that comes and goes.
Rarely, breast cancer can develop in this axillary tail tissue. This is considered a distinct location from the main breast and is uncommon, but it’s one reason a persistent lump in the armpit warrants evaluation, particularly in women.
What a Lump’s Characteristics Tell You
Pain is actually somewhat reassuring. Painful, tender lumps are more likely to be infections, cysts, or inflamed lymph nodes. Cancerous lumps in the armpit tend to be painless, hard, and fixed in place, meaning they don’t slide around when you push on them. They may also feel irregular in shape rather than smooth and round.
That said, physical characteristics alone can’t reliably rule cancer in or out. What matters more is the overall picture. Risk factors that raise concern for malignancy include being over 40, having a lump that lasts longer than four to six weeks, experiencing unexplained fevers, night sweats, or weight loss, and having swollen nodes in more than one region of the body. A lump that hasn’t returned to its baseline size after eight to 12 weeks also deserves further investigation.
One study of patients who had ultrasound-guided biopsies of abnormal armpit lymph nodes found that among those who presented with an armpit mass, 26% turned out to be malignant. But among women whose abnormal node was found incidentally or alongside low-suspicion breast findings, the malignancy rate dropped to just 1%. Context matters enormously.
How Armpit Lumps Are Evaluated
A doctor will start with a physical exam, checking the lump’s size, texture, mobility, and tenderness. They’ll also feel for swollen nodes in other areas like the neck and groin, and ask about recent infections, vaccinations, and symptoms like fever or weight loss.
If the lump doesn’t have an obvious explanation, ultrasound is typically the first imaging step. It can distinguish between fluid-filled cysts, solid masses, and enlarged lymph nodes, and it gives a detailed picture of the node’s internal structure. For women, a mammogram may be ordered alongside the ultrasound to check for related breast findings.
When a lump looks suspicious on imaging, the next step is a needle biopsy. This involves inserting a thin needle into the lump, usually guided by ultrasound, to extract cells for examination under a microscope. The procedure is done with local anesthesia and typically takes only a few minutes. Results usually come back within a few days and provide a definitive answer about whether the tissue is benign or malignant.
When to Pay Attention
Most armpit lumps that appear suddenly alongside other signs of illness, like a cold, sore throat, or skin infection, will resolve as the illness clears. A lump that shows up after shaving or after a vaccine has a straightforward explanation. The lumps worth watching are the ones that persist beyond two weeks, feel hard or immovable, keep growing, or come with systemic symptoms like unexplained weight loss, drenching night sweats, or persistent fevers. A lump that comes back after being removed also warrants a closer look.