The therapeutic effectiveness of radiation from a 311 nm ultraviolet B (UVB) lamp (Philips TL-01) in a near vs. far erythemogenic therapy regimen was investigated in 13 patients with widespread, symmetrically distributed psoriasis. The patients received UV therapy starting with 70% of the 311 nm minimal erythema dose (MED) on one randomly chosen half of the body and 35% of the 311 nm MED on the other half. Therapy was given three to five times a week, and the UVB dose in both regimens was increased simultaneously in the same relation. For the 11 patients completing the study, the mean psoriasis area and severity index (PASI) score for the near vs. far erythemogenic treatment side was 21.2 vs. 18.5 before therapy (Wilcoxon's test, not significant), 11.8 vs. 14.4 at week 1 (P = 0.003), 8.2 vs. 12.0 at week 2 (P = 0.004), and 6.6 vs. 15.6 at week 3 (P = 0.005). After 3 weeks, a satisfactory response (i.e. improvement of the initial PASI score by more than 75%) was observed in six of 11 patients on the near erythemogenic treatment side vs. three of 11 patients on the far erythemogenic side. However, the definitive median total number of treatments needed to achieve a satisfactory therapy response on the near vs. far erythemogenic sides was 12 vs. 16 (P = 0.022), whereas the definitive median cumulative UV dose was 14.0 vs. 9.1 J/cm2 (P = 0.088), respectively. These results suggest that near erythemogenic 311 nm UVB therapy may clear psoriasis faster than far erythemogenic therapy but that the latter regimen may be equally effective as it requires slightly more treatment sessions at a lower (and possibly less carcinogenic) cumulative UV dose.