We have examined the effects of a standardized, moderately erythemogenic dose of long-wave ultraviolet (UVA) radiation on normal human skin, with the use of an appropriately filtered solar simulator and sequential biopsy specimens processed as 1-μm Epon-embedded sections. Histologic changes were present immediately after irradiation and evolved slowly during the 48-hour study. The epidermis manifested slight intracellular and intercellular edema and progressive loss of Langerhans cells to approximately one-fifth control values. A dermal infiltrate of neutrophilic polymorphonuclear leukocytes was present in all postirradiation specimens and peaked at 3 hours. A perivascular lymphocytic infiltrate, moderate endothelial cell enlargement, mast cell hypogranulation, occasional massive venular dilation, and sparse red blood cell extravasation were also noted. Overall, our findings expand and quantify earlier impressions that, compared to UVB, UVA has a relatively greater histologic effect on the dermis than on the epidermis, depletes epidermal Langerhans cells, and recruits neutrophils into irradiated human skin.