Small red pimples on the arm may be related to acute eczema, keratosis pilaris, insect bite dermatitis and other diseases. 1. Acute eczema: acute eczema can be manifested as erythema in the affected area, often accompanied by varying degrees of itching, and after the erythema will appear pimples or small blisters, when it develops on the arm, the patient can appear on the arm of the symptoms of small red pimples. 2. Peri-corneal keratosis: Peri-corneal keratosis is mainly manifested as keratinized papules at the hair follicles and erythema around the hair follicles, and when it develops on the arm, patients can appear on the arm with small red pimples. 3. insect bite dermatitis: insect bite dermatitis can occur in exposed parts of the human body, such as the head and face, limbs, etc., the affected areas often appear in clusters or scattered wind cluster-like erythema lesions, when it develops on the arm, the patient can appear on the arm with small red pimples. There are other causes of small red acne on the arm, it is recommended that patients go to the hospital in time to find out the cause of the disease, standardized treatment, so as not to delay the condition.