A 4-year follow-up study of 631 patients with psoriasis given 8-methoxypsoralen and UVA in the PUVA-48 cooperative study revealed only ten patients with skin cancer and eight patients with actinic keratosis. The skin cancer group averaged 14 years older than the group without lesions. The cumulative treatment ranged from 255 to 1,737 joules/cm2 at the time of development of skin cancer. The ratio of basal cell carcinoma to squamous cell carcinoma and the anatomic location of lesions generally follow the pattern of spontaneously developing lesions. Ionizing radiation was a significant risk factor, but previous arsenic exposure or skin cancer was not. Comparison with previous studies and lack of control population may indicate less risk of skin cancer following PUVA than previously suggested.