Small red dots on the arm are painless and itchy, and the possible clinical causes are vascular nevus, allergic purpura, peripheral keratosis, and so on. 1. Vascular nevus: usually caused by abnormal development of vascular structure or abnormal proliferation of vascular endothelial cells, clinically manifested as dark red or burgundy dots on the arm, with the appearance of slightly elevated nodular superfluous organisms. 2. Henochromatosis purpura: the cause of the disease is more complex, usually associated with viral infections, drug side effects, food allergies and other factors, the clinical manifestations of the affected areas of the red rash, the state of the mild disease is not obvious pain and itching. 3. perifollicular keratosis: a chronic follicular keratosis pilaris, with the lack of vitamin A, metabolic disorders and other factors related to the clinical manifestations of the upper arm and other parts of the symmetrical distribution of red pimples, usually no itching and other discomforts. Arm long red dot painless itch may also be other reasons, it is recommended to go to the hospital dermatology department, clear diagnosis for targeted treatment.