A lump in the armpit may be caused by lipoma, sebaceous cyst, or swollen lymph nodes in the armpit. 1. Lipoma: Lipoma is a proliferating lump of fat that can occur anywhere there is fat for reasons that are unclear. Lipoma occurs in the armpit, resulting in a localized raised mass. 2. Sebaceous cysts: The sebaceous gland ducts are blocked and the secretions cannot be discharged and collect locally to form cysts. Small cysts may not have any symptoms, but if the cysts gradually increase in size, they may cause the skin surface to bulge, causing pain when pressed, and redness, swelling and pus flow when combined with infection. 3. Swollen lymph nodes in armpit: bacterial infection, tuberculosis, virus or metastatic cancer in lymph nodes may lead to swollen lymph nodes in armpit, and localized lumps may appear. It is recommended not to treat the lumps in the armpit by yourself, but go to the dermatology department of the hospital to look for the cause first, and then treat the primary disease.