Four hundred eighteen patients were treated with oral photochemotherapy up to 5 years and monitored regularly for the occurrence of precancerous conditions or tumors of the skin. Out of this group, six patients (1.4%) developed actinic keratoses and five (1.2%), epidermal tumors (three squamous cell carcinomas, one basal cell carcinoma, one keratoacanthoma) 12 to 53 months after initiation . of 8-methoxypsoralen-photochemotherapy (PUVA). Ten of these eleven patients belonged to a group of 172 individuals with a history of previous exposure to arsenic, ionizing radiation, and/or methotrexate who were considered a risk group. All of the four carcinoma patients had previously had one or more courses of arsenic therapy. No tumors were observed in the remaining 246 nonrisk patients. The mean age of the keratosis-tumor group (59 ± 9 years) and the total cumulative long-wave ultraviolet light (UVA) dose (1,045 ± 959 J/cm²) were significantly higher (p < 0.01) than the mean age (45 ± 16 years) and the total UVA dose (658 ± 653 J/cm²) of the 407 patients without such skin changes. The skin type and the time period between initiation of PUVA therapy and the final evaluation of the patients did not exhibit substantial differences in both groups. The observed incidence of epidermal carcinomas in the risk group (2.3%) was considerably higher than the expected age-sex-specific incidence of a randomized population (0.1%), whereas the incidence in the nonrisk group (0%) corresponded to the expected rate. However, data on the tumor incidence in psoriatics not treated with PUVA and matched for previous treatments and risk factors were not available. This study shows that the incidence of nonmelanoma skin tumors in long-term PUVA patients without previous medication of arsenic or treatment with ionizing radiation does not differ from the expected incidence of the general population. However, there is an increased incidence of tumors in PUVA patients if certain risk factors are present. The risk factor in our series is previous medication with arsenic, which is carcinogenic per se.