Causes of hard bumps on arms include flat warts, moles, dermatofibromas, etc. Treatment includes laser therapy, medication and surgery. 1. Flat warts: If the hard bumps are caused by human papilloma virus infection, physical methods such as liquid nitrogen freezing or carbon dioxide laser treatment are recommended to remove the lesions. You can also choose imiquimod cream and other treatments. 2. Moles: If the hard lumps are moles, laser or surgical excision treatment can be given at this time. 3. Dermatofibroma: For hard lumps caused by fibrous hyperplasia, surgical excision can be given at this time. There may be other reasons for the hard lumps on the arm, and it is recommended that patients with similar symptoms seek timely medical treatment by a professional doctor for standardized diagnosis and treatment.