Psoralens have been used previously in an attempt to repigment vitiliginous skin. Recent reports indicate psoralens, activated by long wavelength ultraviolet light (black light), inhibit DNA synthesis in various cell types. This suggested to us a possible new therapeutic use for psoralens in proliferative epidermal diseases such as psoriasis. This study shows that trioxsalen plus black light markedly inhibits epidermal DNA synthesis. Trioxsalen or black light alone has no significant effect. Our own preliminary clinical studies show improvement of chronic plaque type psoriatic lesions in eight of 11 patients using topically applied trioxsalen and black light. We suggest that further critical clinical investigation of psoralens either topically or systemically in combination with black light for proliferative skin diseases is warranted.