The effect of UVB therapy after short contact dithranol therapy (SCDT) was examined in 53 patients with psoriasis. After dithranol application, patients were randomly allocated to receive either a tar bath and UVB therapy (27 patients) or an emulsifying ointment bath only (26 patients). Forty eight patients completed the study; 16 of the 21 dithranol only patients cleared in a mean of 19.5 days compared with 20 of the 27 dithranol + UVB patients who cleared in a mean of 20.3 days. These differences were not significant. Similarly there was no significant difference in response between the dithranol only and dithranol UVB group as measured by the change in plaque thickness measured using Harpenden calipers, and the severity of psoriasis assessed by the extent of the rash, the degree of scaling, redness and induration. Thirteen of the 16 dithranol only patients who cleared relapsed in a mean of 10.6 weeks compared with 14 of the 20 dithranol UVB patients who relapsed after a mean of 18.9 weeks (P less than 0.05). Comparison of pre- and post-treatment full blood count, biochemical screen and urinalysis showed no evidence of systemic toxicity due to SCDT. This study shows that UVB therapy does not improve the clearance of psoriasis in SCDT, but does significantly postpone relapse.