Background: Long-term oral 8-methoxypsoralen (8-MOP) and UVA (PUVA) therapy increases the risk of nonmelanoma skin cancer and possibly also of cutaneous malignant melanoma. Topical application of 8-MOP PUVA induces malignant tumors in rodent skin, but little is known about its carcinogenicity in human skin. Objective: Our purpose was to investigate the carcinogenicity of 8-MOP bath PUVA in humans. Methods: This was a cohort study of 158 patients with psoriasis, for whom 8-MOP bath PUVA had been initiated during 1979 to 1992. The average number of 8-MOP bath PUVA treatments was 36 (range, 6 to 204) and the mean cumulative UVA dose was 92 J/cm² (range, 3 to 884 J/cm²) by the end of 1995. The patients were not treated with any other forms of PUVA. Cancer incidence subsequent to 8-MOP bath PUVA up to the end of 1995 was determined by linking the cohort with the records of the Finnish Cancer Registry. The standardized incidence ratios (SIR) were calculated for skin cancer and some common internal cancers, using the expected numbers of cases based on the regional cancer incidence rates. Results: There was one case of basal cell carcinoma, but no cases of other types of skin cancer. A total of 6 noncutaneous cancers were observed (SIR, 1.3; 95% confidence interval, 0.5 to 2.8). Conclusion: No association between cutaneous cancer and 8-MOP bath PUVA was found, but the statistical power of this study alone is not adequate to warrant definite conclusions. The results can be used in a meta-analysis as soon as other studies on the carcinogenicity of 8-MOP bath PUVA are published.(J Am Acad Dermatol 1999;40:694-6.)