A hard lump inside the armpit may be caused by sebaceous cyst, lymphadenitis, lymphoma and so on. 1. Sebaceous cysts: sebaceous gland secretion can not be discharged from the mouth of the gland in a timely manner, resulting in sebum stagnation and the formation of cysts, which can be manifested as a hard lump in the armpit, and redness, swelling, and pain can be seen when secondary bacterial infection occurs. 2. Lymphadenitis: acute or chronic inflammation in the drainage area of axillary lymph nodes involves the lymph nodes, resulting in lymph node enlargement, which can be manifested as a hard lump in the armpit. 3. Lymphoma: malignant tumor originated from human immune system, occurring in armpit can also be manifested as axillary mass, usually not accompanied by pain. There may be other reasons why there is a hard lump in the armpit, so we suggest you go to the hospital in time, clarify the cause of the disease, and carry out standardized treatment under the guidance of the doctor.