In a double-blind left-right randomised comparison, 27 patients suffering from chronic plaque-type psoriasis vulgaris were treated for one minute with dithranol 2% ointment, Psoralon (Psoralon MT), on a selected psoriasis plaque on one half of the body and with a placebo ointment on a corresponding plaque on the other. The preparations were applied once daily for 8 weeks. Seventeen patients achieved clearing or considerable improvement with dithranol therapy, as compared with 6 patients with placebo (p = 0.002). Erythema, infiltration, scaling, pruritus and the overall result were assessed. Statistically significant differences in favour of dithranol treatment were seen for all five variables, except for pruritus. The average of these five variables, designated the mean score, was also analysed; dithranol was seen to yield significantly better results (p = 0.001). Staining of clothes and the bathroom was noted by 3 and 5 patients, respectively, but no medical side effects were seen.