A hard lump at the back of the waist may be caused by sebaceous fibroma, boils, fasciitis and other reasons. 1. Sebaceous fibroma: when the fibroblasts or local tissue cells at the back of the waist have abnormal proliferation causing benign tumors in the dermis, if the tumors become diseased and press on the local nerves or blood vessels, the symptom of a hard lump at the back of the waist will appear. 2. Boils: when the hair follicles behind the waist and the surrounding tissues are caused by Staphylococcus aureus, causing red, swollen, hot and painful nodules, which can be manifested as a hard lump behind the waist and pain symptoms. 3. Fasciitis: when the local muscle tissue behind the waist is cold or long time friction caused by muscle surface fascia radiating pain nodules, manifested as a hard lump behind the waist pain symptoms. Because of the causes of a hard lump behind the waist pain more causes, should also go to the hospital in time for examination, after a clear cause, under the guidance of a professional doctor for targeted treatment, to avoid the impact on the health of the body.