A hard bump on the back of the left hand may be associated with warts, dermatofibromas and epidermal cysts. 1. Warts: due to the back of the hand skin damage caused by infection with human papilloma virus, the main clinical manifestation of one or more wart-like bumps, the surface of the cauliflower pattern or papilloma-like, hard, occasionally itchy. 2. Dermatofibroma: a benign tumor formed by the proliferation of subcutaneous fibers due to the injury of the skin on the back of the hand, often manifested as a single, hard nodule or papule, usually without conscious symptoms. 3. Epidermal cysts: this disease may be related to epidermal injury, embryonic ectopic factors, or may be caused by the destruction of the hair follicle structure. The disease can occur in all parts of the human body, mainly manifested as subcutaneous nodules, mostly round, oval, etc., hard texture, a few millimeters to several centimeters large. There are many reasons for the hardness of the bag on the back of the left hand, so it is recommended that the patient go to a regular hospital and have a relevant examination to determine the cause of the disease.