The appearance of many red dots on the arm the size of a needle tip to consider a variety of possibilities, may be purpura, keratosis pilaris, multiple folliculitis, allergic dermatitis and other diseases. 1. Purpura: common such as allergic purpura, thrombocytopenic purpura, etc., can appear in the local skin dense dots or patches of bruises, petechiae, etc.. 2. Perifollicular keratosis: Perifollicular keratosis can be manifested as symmetrically distributed pinhead-sized follicular papules on the outside of the limbs, mostly on the upper arms, mostly in adolescence or pre-pubertal episodes, mainly related to genetic factors. 3. multiple folliculitis: folliculitis can be single or multiple, the early symptoms of multiple folliculitis can be manifested as pinpoint-sized red papules, which are not accompanied by pain, and pus will appear locally in the later stage, which is accompanied by pain. 3. Allergic dermatitis: Allergic dermatitis can be manifested as dense red spots or patches on the local skin, the color is usually bright red, which can be accompanied by different degrees of itching. There are many causes of atopic dermatitis in the arm area, such as diet, medication, local contact with sensitizing substances and other factors. In addition to the above reasons, congenital skin capillary development abnormalities, spider nevus, etc. can also be manifested as a lot of red dots on the arms of the needle tip size, you need to consult a dermatologist to clarify the diagnosis, and to give targeted treatment.