BACKGROUND Narrowband ultraviolet B (NB-UVB) phototherapy is an effective treatment for psoriasis. OBJECTIVE To compare the effects of three and five times weekly NB-UVB phototherapy in the treatment of chronic plaque psoriasis. METHODS Sixty-five patients with chronic plaque psoriasis were allocated to receive three or five times weekly NB-UVB, starting at low dose. RESULTS Among the patients who completed the study, clearance was achieved in 18 out of 23 patients (78%) in the three times weekly group and in 15 out of 22 patients (68%) in the five times weekly group. The difference was not statistically significant (P=0.44). No statistically significant differences were found between the two groups in the number of treatments (P=0.95), cumulative UVB dose (P=0.51), and rate of side-effects. Length of the treatment period was significantly shorter in the five times weekly group (P<0.001). At the end of treatment, the mean psoriasis area and severity index score was lower in the three times weekly group (P=0.02). CONCLUSIONS We recommend three times weekly NB-UVB for chronic plaque psoriasis; however, the more rapid clearance of psoriasis with five times weekly phototherapy may justify using this method in some patients.