From 1991 to 1992, 28 patients with chronic plaque psoriasis were treated with a new UVB treatment regimen guided by skin reflectance measurements. The severity of the disease, the efficacy of the treatment, the number of side-effects and the cumulative UVB dose were recorded. These data were compared in an open study with the data obtained from 18 patients with chronic plaque psoriasis, who were treated from 1988 to 1991 at our department with a UVB treatment regimen with regular dose increments. There was no difference in the severity of the disease at the beginning of the therapy or in the healing score between the two groups. The cumulative UV exposure was lower with the new treatment regimen (30 basic minimal erythema doses) compared with the old regimen (89 basic minimal erythema doses). The skin reflectance-guided UVB therapy achieved a response earlier (5 weeks) than the regimen with stepwise dose increments (9 weeks). This study indicates that skin reflectance-guided UVB phototherapy may enable the dermatologist to lower the cumulative UVB exposure dose without reducing the effect of UVB phototherapy in the treatment of psoriasis. The results of this open retrospective study emphasize the need to conduct a prospective double-blind controlled study in the future.