What is an underarm lump?
An underarm lump is a rounded mass that appears under the skin in the armpit. If a lump is large, it can appear as a visible bulge, and if it is small, you may be able to detect it only with the fingers.
A painful lump in one or both armpits is usually due to infection.
A painless lump in the armpit can be due to a cyst or cancer.
In women, a hard, painless lump in one armpit is suspicious for breast cancer.
There seem to be no conditions that would appear with lumps exclusively in the right or left armpit.
Small bumps that appear on the surface of the underarm skin are described in the article about armpit rash.
Painful Armpit Lumps
Painful armpit lumps are usually either swollen lymph nodes or abscesses (collections of pus). They are usually greater than 1 cm.
Systemic infections with lumps in both armpits
In certain systemic infections, which affect the entire body, the lymph nodes in the armpits become swollen and tender, sometimes with reddened overlying skin .
In infectious mononucleosis (infection by the Epstein-Barr virus), which mainly occurs in young adults, swollen lymph nodes usually appear in the neck, in both armpits and sometimes in the groin. Other symptoms include fever, severe fatigue, excessive sweating, sore throat .
In toxoplasmosis, which is a parasitic infestation contracted by ingestion of cat feces, painful swollen lymph nodes appear in the neck and both armpits; other symptoms include sore throat and mild fever [5,21].
Bubonic plague (mainly in Africa) and rat-bite fever (mainly in Asia) are bacterial infections that can cause sudden fever and very tender lymph nodes in the neck and both armpits [6,11].
Local infections and bites with lumps in one armpit
A bacterial infection or venomous bite in the arm, hand or fingers can result in an infection of the lymphatic vessels, which appear as tender red streaks traveling along the inner side of the arm and with swollen painful lymph nodes in an armpit . Causes include:
- Cat scratch disease after a cat scratch, bite or lick 
- Tularemia (rabbit fever, deer-fly fever) after a deer fly or tick bite 
- Staphylococcal (staph) or streptococcal (strep) skin infections, which can appear as a boil (red, tender bump), cellulitis (red, thickened skin area) or impetigo (skin redness and peeling) [3,4]
- Snake, spider or scorpion bite 
Armpit abscess is a collection of pus under the skin, which can occur as a complication of furuncle, hidradenitis suppurativa, injury or surgery. It appears as a tender, pea- to golf ball-sized lump [9,13].
Clogged milk ducts during breastfeeding
During breastfeeding, some women notice one or more painful or painless lumps in one or both armpits. Such lumps are most likely clogged milk ducts that extend from the breast into an armpit. The lumps can disappear after a short massage or spontaneously with time.
Painless Armpit Lumps
In some healthy skinny individuals, normal lymph nodes in the armpits can be detected by the fingers as soft, non-tender pea-sized nodules .
Conditions with painless underarm lumps:
- An epidermoid cyst appears as a pea- or marble-sized lump, which can have an opening that allows squeezing white greasy material out of it .
- Lipoma is a benign tumor composed of fat tissue; it appears as a soft lump.
- In some viral (finger herpes, measles , shingles , HIV/AIDS [6,20]), bacterial (tuberculosis, brucellosis, syphilis [6,18]) and fungal infections (histoplasmosis, sporotrichosis ), painless lymph nodes appear in both armpits.
- Rheumatoid arthritis , systemic lupus erythematosus (SLE) , Sjögren’s syndrome, sarcoidosis [6,33] and other autoimmune diseases can present with enlarged, painless lymph nodes in the neck, chest, armpits and groin, joint pain and skin rash [3,4,8,31].
- Allergic reaction to sulfa drugs, iodine, or penicillin can cause an itchy rash and non-tender lymph nodes in the armpits [3,4].
Cancerous armpit lumps
Various cancers can appear with multiple, enlarged, fixed, hard and usually non-tender lymph nodes in one or both armpits [3,4].
- Lymphoma can cause dry cough, low-grade fever and fatigue [15,23,24].
- Melanoma – a highly malignant skin cancer – appears as a dark (black, brown or blue) mole about 1 cm in size .
- Leukemia – a cancer of white blood cells can appear with fatigue, paleness, frequent infections, enlarged lymph nodes in the neck, armpits and groin, and enlarged spleen .
- Advanced lung cancer can cause a dry cough, coughing up blood, chest pain and enlarged lymph nodes in the armpit on the affected side .
Painless armpit lumps in women
In women with diabetes type 1, diabetic mastopathy appears with painless hard lumps in one or both breasts or armpits .
Breast cancer usually presents with a non-tender lump in a breast and one or more enlarged hard and painless lymph nodes above the clavicle and in the armpit on the affected side .
Some women with silicon breast implants can have enlarged non-tender lymph nodes [5,30].
Painless armpit lumps in children
Infantile papular acrodermatitis or Gianotti-Crosti syndrome in children between 6 months and 12 years of age appears with painless, mildly enlarged lymph nodes in the armpits and groin, and skin rash on the legs, arms and face, which persists for several months. Causes include [4,6,7,12]:
- Viral infections: cytomegalovirus (CMV), hepatitis A, B or C, herpes virus, infectious mononucleosis, parvovirus B19, rubella
- Bacterial infections: cat scratch disease, Lyme disease
- Vaccines against diphtheria, hepatitis B, influenza, measles-mumps-rubella, smallpox, pertusis and polio
- Mononucleosis MedlinePlus
- Lymphangitis MedlinePlus
- Partridge E, Lymphadenitis differential diagnosis Emedicine
- Axillary (arm pit) lymph node enlargement Doctors Longue
- Swollen lymph nodes Merck Manual
- Narita M, Lymhadenopathy Antimicrobe
- Infantile papular acrodermatitis DermNet NZ
- Sarcoidosis Cleveland Clinic
- Armpit conditions WebMD
- Orenstein WA et al, 2004, The Clinical Significance of Measles: A Review The Journal of Infectious Diseases
- Plague World Health Organization
- Gianotti-Crosti syndrome clinical presentation Emedicine
- Abscess WebMD
- Snake Bites John Hopkins Medicine
- Alcohol related pain and Hodgkin’s disease OncoLink
- Cat scratch disease Centers of the Disease Control and Prevention
- Tularemia Centers of the Disease Control and Prevention
- Syphilis DermNet NZ
- Shingles (Herpes zoster) NetDoctor
- Lymph node swelling– HIV vs non-HIV enlargement The Body
- Toxoplasmosis NetDoctor
- Sebaceous cyst MedlinePlus
- Signs and symptoms of Hodgkin disease Cancer.org
- Signs and symptoms of non-Hodgkin lymphoma Cancer.org
- Dan’s story: If only he had known Skincheck.org
- Number stages of breast cancer Cancer Research UK
- Understanding leukemia – symptoms WebMD
- What advanced lung cancer is Cancer Research UK
- Christiaensen E et al, 2009, Axillary lymphadenopathy as a first symptom of diabetic mastopathy PubMed Central
- Vaamonde R et al, 1997, Silicone granulomatous lymphadenopathy and siliconomas of the breast PubMed
- Kitridou R, Swollen lymph nodes due to rheumatoid arthritis? Everyday Health
- Shrestha D et al, 2013, Systemic lupus erythematosus and granulomatous lymphadenopathy BMC Pediatrics
- Sarcoidosis Cedars-Sinai