A lump in your armpit is almost always a swollen lymph node, and most of the time the cause is something routine like an infection, a recent vaccination, or irritation from shaving. Your armpits contain 20 to 40 lymph nodes, small bean-shaped glands that filter fluid and trap bacteria and viruses. When your immune system kicks into gear, these nodes can swell enough to feel like a distinct lump under the skin.
That said, not every armpit lump is a lymph node, and not every swollen lymph node is harmless. Understanding what’s behind yours starts with paying attention to how it feels, how long it’s been there, and what else is going on in your body.
Infections Are the Most Common Cause
Swollen lymph nodes in the armpit most often trace back to an infection or injury somewhere in your arm, hand, chest, or breast. A cut on your finger, an infected hangnail, or even a mild skin infection can trigger the lymph nodes on that side to swell as they work to filter out bacteria. Upper respiratory infections, mono, and other viral illnesses can also cause generalized lymph node swelling that you notice most in your armpits or neck.
These reactive nodes typically feel soft, moveable, and tender. They may be the size of a pea or a marble. The swelling usually peaks over a few days and resolves within two weeks as the underlying infection clears. If you can trace the timing back to a cold, a scratch, or a sore throat, that’s a strong clue the lump is your immune system doing its job.
Vaccines Can Cause Weeks of Swelling
If you recently received a vaccination in your arm, particularly a COVID-19 or flu shot, an armpit lump on that same side is a well-documented side effect. The swelling typically appears within the first week after the shot, with a median onset of about six days. What catches many people off guard is how long it can last. Research published in the American Journal of Roentgenology found that vaccine-related lymph node swelling took an average of 102 days to fully resolve after a booster dose, and even longer (around 129 days) after a first dose in a primary series.
This type of swelling is harmless and simply reflects your immune system responding to the vaccine. But because it can persist for months, it sometimes gets flagged on mammograms or other imaging, leading to unnecessary worry. If you have a screening scheduled, it helps to mention any recent vaccinations.
Lumps That Aren’t Lymph Nodes
Not every bump in the armpit involves your immune system. Several other structures can create noticeable lumps in that area.
- Cysts form when a hair follicle or oil gland gets blocked. They feel round, smooth, and slightly squishy, and they sit just under the skin. Most are painless unless they become infected.
- Lipomas are fatty growths that feel soft and rubbery and move easily when you press on them. They grow slowly and are almost always benign.
- Hidradenitis suppurativa is a chronic skin condition that starts with painful, pea-sized lumps under the skin in areas where skin rubs together, including the armpits. These lumps can persist for weeks or months, and some eventually break open and drain pus. The condition is linked to blocked hair follicles and may be connected to hormones, genetics, smoking, or excess weight. Over time, it can create tunnels under the skin that heal slowly and leave scars.
The texture and behavior of the lump often points to its origin. A soft, moveable lump that’s been there for months without changing is more likely a lipoma or cyst. A painful lump that appeared suddenly alongside redness or warmth suggests infection or an inflamed cyst.
When an Armpit Lump Could Signal Cancer
Cancer is the concern most people are searching about, and it’s important to put it in perspective. The vast majority of armpit lumps, especially in younger people without other symptoms, turn out to be benign. In one study of 594 patients who underwent biopsy of a suspicious armpit lymph node, 37.7% were malignant. But that study specifically selected nodes that already looked suspicious on imaging or physical exam, so the real-world rate for the average person who notices a lump is considerably lower. Among screening-detected nodes in patients with no cancer history and only mild changes, just 4.3% turned out to be malignant.
When breast cancer spreads, the armpit lymph nodes are the first place it typically goes. During an exam, your doctor will feel for enlarged nodes, though cancer can be present in normal-feeling nodes and absent in enlarged ones. Lumps associated with cancer tend to feel hard, fixed in place (not easily moved with your fingers), and painless. They also tend to grow steadily rather than fluctuating in size.
Lymphoma, a cancer of the lymphatic system itself, can also present as a painless armpit lump. Systemic symptoms like unexplained weight loss, drenching night sweats, persistent fevers, and fatigue alongside a growing lump are red flags that warrant prompt evaluation.
What the Lump Feels Like Matters
Paying attention to the physical characteristics of the lump gives you useful information to share with a doctor and helps you gauge how urgently you need to be seen.
- Soft, moveable, tender: Most consistent with a reactive lymph node responding to infection or irritation. This is the most common scenario.
- Soft, rubbery, painless, slow-growing: Suggests a lipoma or benign cyst.
- Hard, fixed, painless: More concerning for malignancy. A node that doesn’t move when you push on it may be anchored to surrounding tissue.
- Painful, red, warm to touch: Points to an infected cyst, abscess, or skin condition like hidradenitis suppurativa.
Size matters too. Lymph nodes that swell to over one centimeter (roughly the width of a pencil eraser) are more likely to be investigated, though size alone doesn’t determine whether something is benign or serious.
How Doctors Evaluate an Armpit Lump
If you see a doctor about an armpit lump, the first step is a physical exam. They’ll feel the lump’s size, shape, and mobility, and check for swollen nodes in other areas like your neck and groin. They’ll ask about recent illnesses, injuries, vaccinations, and any systemic symptoms like weight loss or night sweats.
If the lump is likely reactive, your doctor may simply recommend monitoring it for two to four weeks to see if it resolves. For lumps that persist, feel unusual, or come with concerning symptoms, the next step is usually an ultrasound. This imaging can reveal the internal structure of the node, including whether it has a normal shape or has lost its typical architecture, a feature that significantly raises the likelihood of malignancy.
When imaging raises questions, a biopsy provides a definitive answer. For armpit lumps, this is often done as an ultrasound-guided core needle biopsy, where a small sample of tissue is removed through a needle and examined under a microscope. This can diagnose or rule out cancer, identify unusual infections, and detect immune system conditions like sarcoidosis. The procedure is relatively quick and done with local numbing.
Signs You Should Get It Checked
Most armpit lumps that appear during a cold and disappear within a couple of weeks don’t need medical attention. But you should have a lump evaluated if it doesn’t go away after two weeks, feels hard and painful, keeps getting bigger, comes back after it resolved, shows up alongside a fever or signs of infection, or suddenly becomes tender when it wasn’t before. A painless lump that has been gradually growing over weeks, especially if paired with unexplained weight loss, night sweats, or persistent fatigue, should be seen sooner rather than later.