In 1974 a new photobiologic principle i.e. light + drug, called photochemotherapy was discovered in Boston and immediately confirmed in Vienna. Psoralen + UVA (PUVA) photochemotherapy has now been applied to the treatment of more than 24 heterogeneous groups of diseases, especially psoriasis and mycosis fungoides. After 24 years of experience in thousands of patients with psoriasis and 23 other skin disorders, virtually the only risk is the development of squamous-cell carcinomas. This risk is low with two exceptions: previous history of treatment with ionizing radiation or inorganic trivalent arsenic, and patients with recalcitrant psoriasis who require continuous treatment for many years. In a recent report from a large USA clinical trial, melanoma developed in a few patients with psoriasis treated with PUVA. This prospective clinical trial did not have a control population, and therefore, the conclusion that PUVA can cause melanoma is tentative.