Armpit Lump or Swollen Lymph Nodes Explained

Quick Answer

An armpit lump is most often the result of enlarged lymph nodes - your immune system's normal response to fighting an infection.
Most armpit lumps are benign and resolve on their own within 1–3 weeks.
A lump that is hard, doesn't move, is growing, or persists beyond a few weeks should always be evaluated by a doctor.
Lymph nodes in the armpit are sometimes the first sign that breast cancer has spread.

What Is an Armpit Lump?

The armpit (axilla) contains a cluster of lymph nodes - small, bean-shaped glands that filter lymphatic fluid and play a key role in the immune response. Under normal circumstances, they are not felt. When they swell, they become noticeable as a lump or bumps under the arm.

Lymph node swelling in the armpit is medically called axillary lymphadenopathy. There are typically 20–40 lymph nodes in each armpit. They can swell for many reasons, from a minor infection to serious illness.

Armpit lumps may also originate from structures other than lymph nodes, including skin, sweat glands, hair follicles, and, in some cases, breast tissue that extends into the armpit.

Common Causes

Infections (most common overall):

Localized infections - a cut, scratch, insect bite, ingrown hair, or skin infection on the hand, arm, or breast can cause nearby lymph nodes to swell; usually resolves when the infection clears
Viral infections - cold, flu, COVID-19, or mononucleosis can cause lymph node swelling
Bacterial infections - cellulitis, infected hair follicles, or mastitis

Skin and sweat gland causes:

Hidradenitis suppurativa - a chronic skin condition causing recurring abscesses and lumps in areas with sweat glands
Sebaceous cysts - blocked skin glands forming round, moveable, non-tender lumps
Lipoma - a benign fatty lump that grows slowly, feels soft, and moves freely under the skin
Ingrown hair / folliculitis - infection of hair follicles from shaving

Vaccination-related:

Vaccines - especially COVID-19 vaccines - can cause temporary lymph node swelling on the side of the injection; this is a normal immune response that typically resolves within 4–6 weeks. Always mention recent vaccinations to your radiologist before breast imaging.

Breast-related causes:

Accessory breast tissue - some women have breast tissue that extends into the armpit and swells during hormonal changes
Breast cancer with axillary involvement - when breast cancer spreads, the axillary lymph nodes are typically the first place it goes; a hard, firm, non-moveable lump in the armpit can be a sign that cancer cells have reached the lymph nodes, and this can sometimes be the first noticeable sign of breast cancer
Inflammatory breast cancer - can cause swollen lymph nodes under the arm or above the collarbone

Other systemic causes:

Lymphoma - cancer of the lymphatic system can cause painless, enlarged lymph nodes in the armpit, neck, and groin, often with night sweats, unexplained weight loss, and fatigue
Autoimmune diseases - lupus and similar conditions can cause lymph node swelling

When Should You Get It Checked?

See a doctor if you have an armpit lump that:

Has been present for more than 2–3 weeks
Is growing larger over time
Feels hard, firm, or fixed (doesn't move when you press it)
Is painless - painless lumps can sometimes be more concerning than painful ones
Is accompanied by unexplained weight loss, fever, or night sweats
Is accompanied by breast changes - a new breast lump, nipple changes, skin changes, or discharge
Appears alongside swollen lymph nodes in other areas (neck, groin, collarbone)
Is accompanied by redness, streaking, or pus (signs of serious infection)
You are postmenopausal

Get seen promptly if: The lump is accompanied by severe pain, spreading redness, fever, red streaking on the skin, or you have known or suspected breast cancer.

How Doctors Evaluate It

History and physical exam - how long it's been present, whether it's changing, recent infections, vaccinations, or illness, size, texture, mobility, tenderness, and surrounding area including the breast
Breast examination - because axillary lymph nodes drain the breast, any armpit lump in a woman should prompt a breast examination
Blood tests - a complete blood count (CBC) may check for signs of infection, anemia, or abnormal blood cell counts
Breast ultrasound - usually the first imaging tool; can distinguish between a fluid-filled cyst, a solid mass, and a swollen lymph node; can also assess the internal architecture of a lymph node for suspicious features
Mammogram - used when a breast cause is suspected, typically in women over 30. It includes part of the underarm (axilla), though only a portion is seen on standard views.
CT or PET scan - used if lymphoma or widespread cancer is suspected, to assess lymph nodes throughout the body
Ultrasound-guided biopsy - if a lymph node looks suspicious on imaging, a needle biopsy provides the definitive diagnosis

What Happens Next?

Infection-related swelling: Treat the underlying infection. Lymph nodes should return to normal within 1–3 weeks.

Lipoma / skin cyst: Observation if small and not causing problems; surgical removal if large or bothersome.

Vaccination-related swelling: Reassurance; resolves within 4–6 weeks. Important to note before any breast imaging appointments.

Lymphoma confirmed: Referral to a hematologist/oncologist for staging and treatment (chemotherapy and/or radiation).

Breast cancer with axillary involvement confirmed: Full staging to determine the extent of disease, followed by multidisciplinary treatment planning. Axillary lymph node status is one of the most important factors in breast cancer staging and treatment decisions.

Common Questions About Armpit Lumps

These are the most common questions patients have about armpit lumps and swollen lymph nodes.

1. I had a COVID vaccine last month and now have a lump in my armpit. Is this cancer?

Almost certainly not. This is a well-recognized, expected immune response to COVID-19 vaccines (and some other vaccines). The lymph nodes on the same side as the injection become temporarily enlarged as your immune system responds. This is a sign the vaccine is working and typically resolves within 4–6 weeks. Always tell your radiologist if you've had a recent vaccination before a mammogram or breast MRI, so the finding isn't mistaken for something serious.

2. Could an armpit lump be the first sign of breast cancer?

Yes, it can be. When breast cancer spreads, the axillary lymph nodes are typically the first place it goes, and swollen lymph nodes in the armpit are sometimes the first noticeable sign of breast cancer - even before a lump in the breast can be felt. This is why any persistent, hard, or growing armpit lump in a woman warrants a breast examination and imaging alongside the lymph node evaluation.

3. My armpit lump is painful - does that mean it's less likely to be serious?

Not necessarily. Painful lymph node swelling is common with infections, and most infections are benign. However, some cancers - including certain lymphomas and inflammatory breast cancer - can also cause painful lymph nodes. The features that matter most are: how long the lump has been there, whether it is growing, whether it is hard and fixed, and whether you have other worrying symptoms. Do not assume a lump is benign simply because it hurts.

Armpit Lump or Swollen Lymph Nodes