Specimens obtained from human skin following the induction of erythema responses with long ultraviolet (UVA) and with middle ultraviolet (UVB) radiation alone were compared histologically to determine whether the effects of these two portions of the electromagnetic spectrum were the same. It was found that, except for similar dermal inflammatory changes, the effects of UVA were not the same as those of UVB; characteristic epidermal sunburn damage was induced by the latter, but not by the former.