A painful hard bump under the armpit may be caused by boils, epidermal cysts, lymphadenitis and other diseases, which can be treated with general therapy, medication or surgery. 1. Boils: If boils are the cause of painful hard bumps under the armpits, pay attention to personal hygiene and wear loose, comfortable clothes. Early non-purulent can follow the doctor’s instructions external ichthyol ointment, iodine tincture and other drugs, if necessary, the systematic use of enzyme-resistant penicillins, cephalosporins, with anti-inflammatory, swelling and other effects. 2. Epidermoid cysts: epidermoid cysts secondary infection can also cause a hard bag under the armpit is painful, need to control inflammation after elective surgical treatment, excision of cysts. 3. Lymphadenitis: a painful hard bag under the armpit may also be caused by lymphadenitis, the patient can follow the doctor’s instructions oral cephalosporin, roxithromycin and other antibiotics to control the inflammation. There are many reasons for the pain in the armpit, it is recommended that the patient to go to the hospital, clear the cause of the disease, and then follow the doctor’s instructions for treatment.