A hard lump on the back may be caused by boils, dermatofibroma, epidermal cysts and other diseases. 1. Boil: Boils may be caused by Staphylococcus aureus, excessive sweating, scratching the skin and other factors, the patient’s skin will appear single or multiple red, hemispherical painful nodules, the disease can occur everywhere on the body surface. 2. Dermatofibroma: the formation of this disease is considered to be related to insect bites, trauma and other factors, the patient’s skin will appear round, ovoid and other solid nodules, which can occur in all parts of the body. 3. Epidermoid cysts: generally manifested as tense bulging, hard cysts, most commonly found in the head and face, upper trunk and other places. There are many reasons why a hard bag grows on the back, so it is recommended that the patient should consult a doctor for examination to clarify the cause of the disease.