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Background: The safety of psoralen plus ultraviolet A (PUVA) light therapy has been an issue of debate. A few multiple-center cooperative studies have reported an increase of basal cell and squamous cell carcinomas among PUVA-treated patients. In our institute, more than 1000 patients have been treated with PUVA since 1975. Objective: We investigated the incidence of skin cancer among patients who received high doses of PUVA to see whether such incidence increased. Methods: This is a historical cohort study of two comparison groups of patients. Subjects under study were 492 psoriasis patients who received PUVA treatments between 1975 and 1989. One group of 103 patients, defined as the high-dose group, received an accumulated PUVA dose of 1000 joules/cm 2 or more; another group of 389 patients, as the low-dose group, received 200 joules/cm 2 or less. The occurrence of skin cancer in the two comparison groups is analyzed. Results: In the high-dose group we observed an increased number of patients with squamous cell carcinoma, keratoacanthoma, and actinic keratosis. We did not see any patients with genital cancer, melanoma, or an increased number of patients with basal cell carcinoma. Conclusion: The risk of squamous cell carcinoma developing in patients who received a high dose of PUVA is confirmed. We speculate a combination of factors, including PUVA, may contribute to this risk.