Three and five times weekly narrow-band TL-01 (311-313 nm) ultraviolet (UV) B phototherapy regimens for chronic plaque psoriasis were compared in a randomized, observer-blinded, half-body, within-patient paired study. Twenty-one patients [13 men, eight women, age range 21-68 years, skin phototypes I (two patients), II (14) and III (five)] entered the study. Sixteen reached clearance or minimal residual activity (MRA) on both sides. Of the other five, three withdrew because they did not reach clearance or MRA on the 5x weekly side by a maximum of 30 treatments, one when he was satisfied with moderate improvement and one because of repeated failure to attend. Those who completed treatment reached clearance or MRA after a median of 35 days with 5x weekly treatment compared with 40 days with 3x weekly treatment (P = 0.007), but required a median of 23.5 compared with 17 UVB exposures (P = 0.001) and 94 minimal erythema dose multiples (MEDs) compared with 64 MEDs (P = 0.01). Fifteen (of 16) developed at least one episode of well-demarcated erythema during 5x weekly treatment compared with just three of 16 treated 3x weekly (P < 0.001). There was no significant difference between regimens in duration of remission. For this skin phototype I-III population, the more rapid clearance of psoriasis with 5x weekly phototherapy is not, for the majority of patients, sufficient to justify the extra exposures and higher UVB dose. We no longer use 5x weekly phototherapy for psoriasis.