Hard bumps on the arm may be keratosis pilaris, folliculitis, flat warts and other diseases caused by different diseases can be selected according to different ointment treatment. 1. Keratosis pilaris: local topical 0.05% to 0.1% retinoic acid ointment, 3% to 5% salicylic acid ointment, 10% to 20% urea cream or 12% ammonium lactate lotion, which can soften or dissolve the keratin, improve the symptoms. 2. Folliculitis: a purulent inflammation limited to the mouth of the hair follicle. Early no pus can be used mupirocin ointment and other antibiotic ointment. 3. flat warts: can use 5% imiquimod ointment, retinoic acid cream and other retinoic acid ointment. It is recommended that people with hard bumps on their arms go to the hospital in a timely manner, to clarify the cause of the disease, under the guidance of the doctor for standardized treatment, not blindly self-medication, so as not to delay the condition.