A lump in your armpit is most often a swollen lymph node reacting to an infection, but it can also be a cyst, a blocked hair follicle, or, less commonly, a sign of something more serious. Your armpits contain a dense cluster of lymph nodes, which are small, bean-shaped structures that filter bacteria and other threats from your upper body. When those nodes activate, you feel them as tender, marble-sized bumps just under the skin.
Swollen Lymph Nodes: The Most Common Cause
The armpit is one of the body’s main lymph node hubs, and infections or injuries anywhere in your arm, chest, or upper back can trigger swelling there. A cut on your hand, a skin infection on your arm, or even a common cold can cause one or more nodes to enlarge as your immune system ramps up. These swollen nodes are typically soft, movable under the skin, and tender to the touch. They tend to swell quickly and shrink back down within a few days to a couple of weeks once the underlying infection clears.
Cat scratch disease is a classic example. After a scratch or bite from a cat carrying the Bartonella bacterium, symptoms start three to ten days later: a rash or small bumps near the wound, followed by painful, swollen lymph nodes in the nearest cluster (often the armpit if the scratch was on your hand or arm). Those swollen nodes can persist for two to eight weeks before resolving, sometimes accompanied by fever, fatigue, and muscle aches.
Cysts and Fatty Lumps
Not every armpit lump involves a lymph node. Cysts are fluid-filled sacs that form beneath the skin. They feel firmer and more rounded than swollen lymph nodes, and they tend to stay fixed in place when you press on them rather than sliding around. Unless a cyst becomes inflamed, it usually isn’t painful. Cysts form slowly and can sit unchanged for months or even years without disappearing on their own.
Lipomas, which are soft lumps made of fatty tissue, can also appear in the armpit. They’re rubbery, painless, and move easily when pushed. Lipomas are almost always benign and grow very slowly. Both cysts and lipomas are generally harmless, though they can be removed if they become uncomfortable or bothersome.
Blocked Hair Follicles and Skin Conditions
The armpit is full of hair follicles, sweat glands, and oil glands, making it a prime spot for skin-related lumps. A simple ingrown hair or infected follicle can produce a red, painful bump that looks and feels alarming but typically resolves on its own or with basic care.
Hidradenitis suppurativa is a more persistent condition worth knowing about. It starts with a single, painful pea-sized lump under the skin that lasts weeks or months. Over time, more bumps may form in areas where skin rubs together or where sweat glands are concentrated: armpits, groin, buttocks, and under the breasts. Some bumps eventually break open and drain pus. Blackheads appearing in small pitted pairs are another hallmark sign. This condition is not caused by poor hygiene and is not contagious. It results from hair follicles becoming blocked for reasons that aren’t fully understood, and it tends to recur without treatment.
Vaccination-Related Swelling
If you recently received a vaccine in your upper arm, a swollen lymph node on the same side is a well-documented and temporary reaction. COVID-19 vaccines, flu shots, and other immunizations can all trigger this. With the Pfizer COVID vaccine, armpit swelling typically appeared two to four days after the shot and lasted an average of ten days. The swelling is one-sided, appearing only on the arm where you received the injection, and it resolves without treatment.
When an Armpit Lump May Signal Cancer
Cancerous lumps in the armpit are far less common than benign ones, but they do occur. Lymphomas (cancers of the lymphatic system) can cause lymph nodes to enlarge without any obvious infection. Breast cancer, lung cancer, melanoma, and several other cancers can also spread to the axillary lymph nodes. When swelling is caused by malignancy rather than infection, the nodes tend to feel hard or rubbery, are often painless, and may feel fixed in place rather than movable under the skin. They persist and gradually enlarge rather than shrinking over time.
Autoimmune conditions like lupus, rheumatoid arthritis, and Sjögren syndrome can also cause prolonged lymph node swelling that doesn’t follow the typical pattern of an infection.
How Armpit Lumps Are Evaluated
A doctor will usually start with a physical exam, noting the lump’s size, firmness, mobility, and tenderness. If the lump is soft, movable, and you have an obvious infection or recent vaccine, that’s often enough for a diagnosis. If the cause isn’t clear, an ultrasound is the typical next step. It can distinguish between a fluid-filled cyst, a swollen lymph node, and a solid mass, and it can reveal details about a lymph node’s internal structure that help determine whether it looks normal or suspicious.
When ultrasound findings are concerning, a fine-needle aspiration may follow. This involves inserting a thin needle into the node, guided by ultrasound, to collect a small tissue sample. The procedure is quick and done in an outpatient setting. Results help confirm or rule out cancer without requiring surgery.
Signs That Need Prompt Attention
Most armpit lumps turn out to be harmless, but certain features warrant a call to your doctor sooner rather than later. Take note if the lump doesn’t go away after two weeks, feels hard and painful, keeps getting bigger, or comes back after being removed. A lump accompanied by fever, unexplained weight loss, drenching night sweats, or new tenderness that develops suddenly also deserves evaluation. The absence of any obvious infection, injury, or recent vaccination in someone with a persistent armpit lump raises the index of suspicion for something beyond a routine immune response.