A hard lump on the buttocks is considered to be the result of boils, sebaceous cysts, lipomas and other diseases. 1. Boils: The etiology of boils is related to Staphylococcus aureus infection, skin trauma, scratching, and poor hygiene practices. Patients may develop small red painful nodules on the buttocks, which may develop into firm nodules accompanied by pain and swelling, manifesting as a hard lump on the buttocks. 2. Sebaceous cyst: When the mouth of sebaceous gland of hair follicle is blocked, sebum accumulates into cysts, which can form sebaceous cyst. Single or multiple cysts may appear on the buttocks. When secondary infection occurs, there are localized redness, swelling, pain and other symptoms, and some patients may have a hard lump on the buttocks. 3. Lipoma: the etiology is not yet clear, it may be caused by heredity, chronic inflammation, abnormal lipid metabolism and other factors. Lipoma can appear in any part of the body, manifested as a localized mass, and patients may appear to have a hard lump on the buttocks. There are many causes of a hard lump on the buttocks, patients can identify the cause and take treatment measures.