Dense dots appear on the arm to consider with the perifollicular keratosis, folliculitis, skin purpura and so on. 1. Keratosis pilaris: a chronic follicular keratosis pilaris, mainly related to heredity, vitamin A deficiency and other factors, if involved in the arm, can be manifested in the arm, especially on the outer side of the upper arm, dense dots and other symptoms, dots on the top of the existence of light brown keratin embolism, the interior contains curly hair. 2. Folliculitis: It involves hair follicles and their surrounding tissues, and is related to bacterial infection. The lesions may initially appear as red follicular papules, and pustules may appear in the center of some of them. When the number of lesions is large, it can lead to the appearance of densely packed dots on the arms. 3. Skin purpura: Thrombocytopenic purpura, anaphylactic purpura, etc. can cause dense red dots on the arm and painless and itchy. Before the onset of low fever, upper respiratory tract infection and other prodromal symptoms, with the development of the disease, part of the purpura can also have a tendency to fusion. Therefore, when there are dense dots on the arm, it is recommended to go to the hospital in a timely manner, after a clear diagnosis, to the cause of the treatment, so as not to delay the condition.