Red patches on the arm may be caused by diseases such as pityriasis rosea, solar dermatitis and cherry angioma. 1. pityriasis rosea: the etiology of this disease is unknown, may be related to viral infections, immune dysfunction, allergies, pregnancy and drugs and other factors, common drugs are interferon, isotretinoin, metronidazole and so on. Rash mainly occurs in the trunk and limbs, can be seen round or oval reddish or yellowish-brown plaques, so there are patients with red plaques on the arm. 2. Solar dermatitis: when the skin receives more than the tolerated amount of ultraviolet radiation, it will cause solar dermatitis, which can be induced by excessive alcohol consumption, skin moisture and other factors, resulting in bright red erythema on the sunburned parts of the patient, therefore, red patches on the arm may be caused by solar dermatitis. 3. Cherry angioma: studies have shown that this disease may be related to aging and gene mutation, which mainly refers to the proliferation and expansion of skin capillaries. The patient’s skin will appear bright red or purplish-red papules and nodules of different sizes, which can be manifested as red patches on the arms. 4. Other: In addition, red patches on the arm may be caused by contact dermatitis, urticaria and other diseases. It is recommended that patients with red patches on the arms go to the hospital for a series of relevant examinations to clarify the specific causes of the disease.