Black spots on arms may be caused by hyperpigmentation, endocrine disorders, eczema and other reasons. 1. Hyperpigmentation: under long-term exposure to sunlight, hyperpigmentation may occur, thus causing black spots on the arms. 2. Endocrine disorders: if patients have long-term mental stress, lack of sleep and other conditions, endocrine disorders may occur. It may cause the production of color spots, which may lead to the phenomenon of black spots on the arms. 3. Eczema: Eczema is a kind of inflammatory skin reaction, under the stimulation of inflammation, the excess melanin produced will be deposited in the epidermis and the superficial layer of dermis, so the phenomenon of black spots on the arm may appear. When the patient arm black spots, it is recommended that timely medical examination, identify the cause of the disease, under the guidance of the doctor to actively cooperate with the treatment.