Hard bumps on the inside of the arm may be caused by epidermal cysts, boils, dermatofibromas and other diseases, which can be improved by general treatment, medication and surgery. 1. Epidermoid cysts: Epidermoid cysts may cause hard bumps on the inside of the arms. This disease is usually treated with surgery, and the appropriate treatment needs to be taken according to the symptoms of the disease. If there is no obvious inflammation in the cyst, the cyst can be removed directly by surgery; if the cyst is obviously ruptured or suppurated, the cyst needs to be incised and drained. 2. Boils: Boils may also cause hard bumps on the inside of the arm, and patients should not squeeze or rub the affected area. If the inflammation is mild, antibiotic drugs such as mupirocin and clindamycin can be applied. If necessary, should follow the doctor’s prescription joint oral amoxicillin, ceftazidime and other antibiotic drugs. 3. Dermatofibroma: Dermatofibroma may also cause hard lumps inside the arm, and dermatofibroma usually requires surgical excision treatment. There are many reasons why there are hard bumps on the inside of the arm, and it is recommended that the patient should consult a doctor for examination and clarification of the cause of the disease, and cooperate with the doctor to carry out standardized diagnosis and treatment.