A possible increase in the risk of skin cancer in psoriatic patients treated with long-term maintenance UVB phototherapy was assessed by comparing the cumulative doses of UVB with the amount of UVB received from sunlight by normal healthy people. The biologically-effective UVB dose (termed UVB(EE) ) was measured using polysulphone film and worn as a badge by individuals with either an indoor or an outdoor occupation during 4 summer months of 1983 in The Netherlands (52 degrees N). The calculated mean annual UV-B(EE) doses were 5.9 J/cm2 for persons with an indoor occupation and 134 J/cm2 for those with an outdoor occupation. The UVB(EE) doses received by psoriasis patients during an initial course of phototherapy, as well as during maintenance treatment, were also estimated and gave a mean value of 22 J/cm2. Mean annual amounts of solar UVB(EE) exposure were calculated and compared with the administered doses of UVB(EE) during maintenance phototherapy. A dose-response model is described in order to estimate the increased incidence of non-melanoma skin cancer associated with such therapy. The cumulative incidence among patients who received maintenance phototherapy for several decades was calculated to be a factor of 2.5 to 7.5 higher than the incidence among individuals with an outdoor occupation.