What can cause a lump to form in the armpit?

Patient: What is the most likely cause for a red, painful, and fairly solid lump in the underarm?

Symptoms: Lump in armpit that is red, painful and fairly solid. It is also sometimes warm to the touch.

Doctor: Thank you for question. It is possible that this swelling is an infected sebaceous cyst. This is caused when a sebaceous gland in the armpit is occluded. Bacteria begin to proliferate in this gland leading to swelling with pus, inflammation and pain. You will likely need to be prescribed a course of antibiotics to treat this cyst, and if it is rather large, it may need to be drained by your doctor. Please have your doctor evaluate this swelling and treat you with antibiotics if necessary.Thank you for consulting AskTheDoctor.com