The effect of therapeutic ultraviolet light (UV) radiation on the skin lesions of histiocytosis X (HX) was studied in two old patients. Histiocytosis X cells in the biopsy specimens were strongly reactive with OKT-6, OK-Ia1, an anti-S-100 protein antibody, and were weakly stained with Leu-3a. Some HX cells outgrown from the explants bore immunoglobulin G receptors (Fc-IgG) and C3 receptors. In accordance with clinical improvement after repeated topical 8-methoxypsolaren (8-MOP) plus UV-A (PUVA) or UV-B radiation, the density of infiltrating HX cells gradually was decreased. The PUVA therapy seemed to be more effective than UV-B radiation in our treatment schedule. Even after repeated phototherapies, however, the reactivity of surface and cytoplasmic antigens related to OKT-6, OK-Ia1, and S-100 protein in the remaining HX cells were the same as in untreated HX cells. Although the exact mechanism remains obscure, satisfactory therapeutic results were obtained in response to the phototherapies. New skin lesions eventually recurred after cessation of the treatments, but such eruptions resolved when additional PUVA was resumed. These studies confirm that HX cells share a battery of cytologic characteristics with epidermal Langerhans' cells (LC) and that repeated phototherapies provide a beneficial effect for skin lesions of HX without adverse reactions.