An armpit lump may refer to the enlargement of at least one of the lymph nodes under your arm. Lymph nodes are small, oval-shaped structures that are located throughout the body’s lymphatic system. They play an important role in your body’s immune system.

An armpit lump may feel small. In other cases, it may be extremely noticeable. Armpit lumps may be caused by cysts, infection, or irritation due to shaving or antiperspirant use. However, these lumps may also indicate a serious underlying health condition.

Seek medical attention if you have an armpit lump that gradually becomes enlarged, is or isn’t painful, or doesn’t go away.

Most lumps are harmless and are usually the result of abnormal tissue growth. However, armpit lumps can be related to a more serious underlying health problem. You should have your doctor evaluate any unusual lumps you have.

The most common causes of armpit lumps are:

  • bacterial or viral infections
  • lipomas (typically harmless, benign fat tissue growths)
  • a fibroadenoma (noncancerous fibrous tissue growth)
  • hidradenitis suppurativa
  • allergic reactions
  • adverse reactions to vaccinations
  • fungal infections
  • breast cancer
  • lymphoma (cancer of the lymphatic system)
  • leukemia (cancer of the blood cells)
  • systemic lupus erythematosus (an autoimmune disease that targets your joints and organs)

Armpit lumps can occur in men and women of all ages. However, a lump under the arm could indicate breast cancer. Women should perform monthly breast self-exams and report any breast lumps to a doctor right away.

Note that breasts undergo hormonal changes during the menstrual cycle and may tend to feel more tender or lumpy during this time. This is considered to be completely normal. For the most accurate results, perform breast self-exams about one to three days after your period ends.

Another potential cause of armpit lumps in women, which tend to also occur near the breast and groin regions, is hidradenitis suppurativa. This chronic condition involves clogging and inflammation near apocrine glands of hair follicles in the skin, commonly causing painful boil-like lumps that fill with pus, leak, and possibly even become infected.

Risks for having this condition include tobacco smoking, family history, and obesity. Although the exact cause is not known, it is thought that possibly hormonal changes of puberty and/or the immune system responding too strongly to the hair follicles becoming clogged and irritated. Men can also have hidradenitis suppurativa, but it is much more common in women.

A thorough physical examination is the first step in diagnosing an armpit lump. Your doctor will ask you questions about any changes in the lump, as well any pain you have in the area.

Palpation, which is examining by feel, is used to determine the consistency and texture of the lump. This method is done exclusively by hand as the doctor gently examines the lymph nodes and surrounding tissues.

In some cases, a physical exam may support the conclusion that the lump probably isn’t harmful. For example, benign lumps, such as lipomas, usually don’t require additional treatment. If a lump is bothersome, however, a doctor can recommend treatment options to remove it.

Based on the results of your physical examination, your doctor may order further testing to rule out infection, allergic reaction, or cancerous changes. Your doctor may order a combination of the following diagnostic tests:

  • complete blood count to measure the number of platelets, red blood cells, and white blood cells in your system
  • breast X-ray (mammogram), which is an imaging test that may allow your doctor to see the lump better
  • MRI or CT scan imaging
  • biopsy, which involves removing a small piece of tissue or the entire lump for testing
  • allergy testing
  • a culture of fluid from the lump to look for infection

The course of treatment your doctor recommends depends on the underlying cause of the lump. Bacterial infections can be treated with oral antibiotics. After several days, the armpit lump should start to disappear as your body and the antibiotic fight the infection. If the lump doesn’t respond to oral antibiotics, you may have to be hospitalized for intravenous (IV) antibiotics.

If your lump is associated with allergies, it should subside once you start medication and learn to avoid your allergy triggers.

In most cases, armpit lumps don’t require any treatment, just simple observation. If your doctor determines this is the case, you can use home remedies such as warm compresses and over-the-counter pain relievers to ease any discomfort. Lumps that don’t require treatment include those associated with:

  • lipomas
  • viral infections
  • fibroadenoma (noncancerous breast lumps)

Hidradenitis suppurativa treatment options may include some of the following:

  • antibiotic therapy
  • bleach bath
  • biologic therapy
  • wound dressings
  • anti-acne therapy
  • surgical treatment
  • lifestyle changes

If your armpit lumps are cancerous, your doctor may refer you to a specialist for further care. Treatment will depend on the type of cancer and what stage you’re in, and it may involve a combination of:

The outlook for an armpit lump depends on its cause. For example, a lump that stems from a self-limited viral infection will likely eventually go away on its own. However, a lipoma, while harmless, usually does not go away on its own. A dermatologist can help you remove it.

The outlook for an armpit lump caused by cancer depends on a variety of factors, including the stage of cancer and whether the tumors have spread to other parts of the body. For the best chance of recovery, it’s important you go to your doctor early on for diagnosis and treatment.

Even if you don’t think the lump is harmful, it’s best to contact your doctor for an accurate diagnosis.