White patches on arm skin may be caused by vitiligo, pityriasis alba, anemic nevi and other diseases. 1. Vitiligo: Vitiligo is a series of clinical symptoms caused by the destruction of melanocytes, resulting in reduced melanin production. It is mostly considered to be caused by factors such as decreased immunity, heredity, and trauma. Patients may appear round, oval or irregularly shaped white patches on the forearms, face, neck, hands, abdomen and other parts of the body, manifested as white patches on the skin of the arms. 2. White furfur: Microbial infection, malnutrition, dry skin, vitamin deficiency, lack of sunlight and other factors may induce white furfur. In the early stage of the disease, there will be slightly elevated erythema on the upper arms, face, neck, shoulders and other parts of the body. When the erythema subsides, it will leave white spots covered with dry, bran-like scales on the surface, which will appear as white spots on the arm skin. 3. Anemic nevus: it is caused by the defects of vascular tissues in the process of development, leading to congenital dysfunction, resulting in the blood vessels in a constant state of contraction, and the local blood flow is reduced, making the skin pale. The patients will mostly have pale white spots with irregular edges, which are good for the limbs, trunk, face and other parts of the body, causing the patients to have long white spots on their arm skin. Patients with arm skin white spots are recommended to go to regular hospitals, under the guidance of doctors, to clarify the cause of the disease, and take treatment measures.