Armpit Lumps: Are They Serious?

by PJ Hamel Patient Expert

Is a lump in your armpit considered a breast lump? If you feel a lump there, how serious might it be? Find answers to your questions here.

Discovering an underarm lump

Have you discovered a lump in your armpit? It could be a sign of breast cancer, for two reasons.

First, breast tissue sometimes reaches up into the armpit region. A lump felt in your underarm could in reality be a lump in your breast; you just never realized your breast extended that far.

Second, the lymph nodes in your armpit filter out any abnormalities (infections, reactions to drugs, cancer) in the same-side chest wall, arm, or breast. So a lump under your arm may indicate that your lymph nodes have identified, and are trying to fight, cancer cells that have reached them from your breast.

When to see a doctor

You should see the doctor if the lump doesn’t disappear on its own within a couple of weeks; or if it appears to be getting worse. That said, don’t panic There are many, many causes of underarm lumps, and the vast majority have nothing to do with cancer.

The most prevalent cause of a lump in the armpit is infection. An infection can be localized to the armpit itself (perhaps a nick from shaving); or it can be more widespread. It’s possible to get a localized infection from using an antiperspirant rather than a deodorant; antiperspirants prevent your sweat glands from releasing germs that can cause infections. Or you may have a boil or abscess just under the skin.

Viral infections, like shingles, chickenpox, infectious mononucleosis, and HIV often cause underarm lumps. You may also get a lump under your arm as a reaction to a vaccination, such as smallpox, typhoid or, rarely, measles/mumps/rubella. Sometimes, an allergic reaction to penicillin or iodine produces an underarm lump.

Cancer vs. infection: how to tell the difference

In general, a lump that’s moveable and painful signifies an infection. And, while a painless lump that’s hard and fixed in place is more likely to be cancer than one that’s painful, softer, and moveable, such a lump doesn’t automatically signify cancer.

In fact, an unexplained lump (i.e., one that’s not quickly identified as being from illness or injury) has about a 4 percent chance of being malignant, if you’re over the age of 40; and about a .4 percent chance of malignancy if you’re under 40.

In addition, a lump that seems to increase in size within a few days is probably due to infection, rather than malignancy. Redness/soreness of the skin is also often a sign of a lymph node infection, not cancer.

Next steps

If the lump persists for a couple of weeks, see a doctor. Be prepared: the doctor will likely ask you the following:

  • When did you first notice the lump?

  • Has it changed in size?

  • Does it hurt?

  • Have you noticed anything that seems to make it worse?

  • Do you have any other symptoms of illness?

  • Are you breast-feeding?

If the answers to these questions don’t shed any light on the situation, the doctor may ask you to have an ultrasound, CT scan, and/or a mammogram, any of which will provide a better view of just what the lump consists of.

You may also need fine needle aspiration, a process that draws fluid from the affected lymph node for examination. Finally, you may need a biopsy, if all the other tests prove inconclusive.

Some final advice

If the lump you feel is painful and soft lump, and maybe the skin around it is red, and it seems to be growing, it’s probably from an infection. You should see the doctor for some antibiotics.

And if it’s a hard, painless lump, one that seems fixed in place (and may feel like it’s connected to other hard lumps); and it seems to be staying the same size day to day (not growing quickly), you should see the doctor to rule out cancer.

You also need to see a doctor about any underarm lump that a) seems to be worsening quickly, or b) doesn’t disappear within a couple of weeks. The only time you don’t need to pursue diagnosis and treatment of a lump in your armpit is if it disappears entirely, and doesn’t return.

PJ Hamel
Meet Our Writer
PJ Hamel

PJ Hamel is senior digital content editor and food writer at King Arthur Flour, and a James Beard award-winning author. A 16-year breast cancer survivor, her passion is helping women through this devastating disease. She manages a large and active online survivor support network based at her local hospital and shares her wisdom and experience with the greater community via