Hard bumps on the arm may be caused by insect bite dermatitis, sebaceous cysts, lipomas, folliculitis and so on. 1. Insect bite dermatitis: the arm is bitten by mosquitoes, localized red and swollen patches. Repeatedly scratching and stimulating the local area may result in the formation of hard bumps. 2. Sebaceous cysts: sebum secretion can not be normally discharged from the sebaceous glands, in the local collection of cysts. If accompanied by bacterial infection, hard lumps may be formed. 3. Lipoma: Lipoma is a slow-growing lump of fat, which usually occurs under the skin of limbs and trunk, and it is a skin tumor lesion, which may lead to local hard lumps. 4. Folliculitis: Folliculitis is related to bacterial infection and may lead to localized itching and tenderness. Folliculitis damage is deeper, and scarring is formed during the healing process, leading to localized hard bumps. The appearance of hard bumps on the arm, it is recommended not to self-treat, to avoid leading to the aggravation of the symptoms. You can go to the dermatology department of the hospital, first to clarify the cause, and then targeted treatment.