Acne on the arm may be related to folliculitis, perifollicular keratosis, eczema and so on. 1. Folliculitis: Folliculitis is a bacterial infectious skin disease involving hair follicles and their surrounding tissues, which is mainly manifested as red papules centered on hair follicles, often accompanied by pain and itching. Therefore, when it occurs on the skin of the upper arm, patients can have the symptom of acne on the arm. 2. Keratosis pilaris: mostly related to heredity, vitamin deficiency, metabolic disorders and other factors, often manifested as follicular keratotic papules, and occasionally prickly itch, can also cause arm acne. 3. Eczema: eczema is mainly manifested as red bumps on the affected skin, these bumps look similar to pimples, and often accompanied by varying degrees of itching, therefore, when eczema occurs on the arm, the patient can also appear on the arm acne symptoms. There are other causes of arm acne, it is recommended that patients go to the hospital in a timely manner to find out the cause of the disease, standardized treatment, so as not to delay the condition.