The patient has a small hard lump under the armpit, and the common clinical causes are as follows.1. The armpit is accompanied by excessive proliferation of fibrous tissue, which can be manifested as local fibromatous changes, and hard lumps with hard texture appear. As fibroma is a benign lesion, for the small size of fibroma without obvious clinical symptoms, it is recommended that patients review the local ultrasound after 2-3 months, closely observe the changes of fibroma, and do not need to perform surgical resection for the time being. If the size of fibroma increases gradually and accompanied by local pain in the process of review, it is recommended that the patient should undergo surgical resection in time.2. Breast malignant tumor or lung malignant tumor, its tumor cells can invade to the axilla along with lymphatic vessels and cause lymph node enlargement, which is manifested as a hard lump in the local area. Patients can be examined by color ultrasound or CT to clarify the primary lesion site as soon as possible. Resection of the primary tumor plus axillary lymph node dissection, combined with chemotherapy and radiotherapy after surgery, can completely remove the hard lump in the axilla.