A hard lump in the armpit may be due to the following diseases, such as enlarged lymph nodes and sebaceous cysts. If an infection occurs in the upper extremity or in the chest wall, it may lead to axillary lymphadenitis and the lymph nodes will be enlarged and a hard lump will appear in the armpit, and there will be pain, usually with pressure pain. If a malignant tumor occurs, such as breast cancer, it may lead to axillary lymph node metastasis and a hard lump in the armpit. If a hematologic disease occurs, such as lymphoma, it may also cause a hard lump to appear in the armpit. If a sebaceous cyst occurs, a hard lump will appear in the armpit, which protrudes from the surface skin and has a black spot in the center. Patients are advised to go to the hospital in a timely manner and need to improve relevant examinations, such as ultrasound and blood routine, to be able to make a clear diagnosis and can be treated accordingly.