Brown spots on the arms may be caused by florid chaff rash, seborrheic keratosis, perifollicular keratosis and other diseases. 1. maculopapular furuncle: the causative agent of this disease is Malassezia, and common factors include heredity, prolonged exposure to high temperature and humidity, malnutrition, and exuberant secretion of skin oil. The skin lesions are brown, light red, or close to the skin color of the dotted rash, so brown spots on the arm may be caused by florid chaff rash. 2. Seborrheic keratosis: the cause of this disease is not clear, clinically considered to be related to skin aging, heredity, genetic mutation, ultraviolet radiation, viral infection, air pollution and other factors. The lesions often appear on the back of the hands, arms, trunk, etc. The color changes from skin color, light brown to black, and the morphology is patchy, plaque, etc. Therefore, the patient’s arms will have brown spots. 3. Perifollicular keratosis: It has been shown that the occurrence of this disease is related to heredity, vitamin deficiency or metabolic disorders, hypothyroidism, dry skin and other factors. The main symptom is that there are normal skin color or light red, light brown papules or keratinous plugs on the outer side of the upper arm and the anterior side of the thigh, which can be manifested as brown spots on the arm. It is recommended that patients with brown spots on the arms go to the hospital to improve the relevant examination to clarify the specific cause.