Lumps under the arm are usually enlarged lymph nodes. While there are many possible causes, many of them go away on their own. Some of the possible causes will not go away on their own and lead to severe disease, and can be cured. Diagnosis is usually made by biopsy, which involves a minor surgical procedure. Part or all of the lump is removed for examination. If the lump persists, you and your doctor should consider biopsy. Other treatments, begun without knowing the cause, are unlikely to be helpful.