The antipsoriatic efficacy, tolerability and safety of calcipotriol ointment was compared with tar in a prospective, right/left randomized, investigator-blinded controlled study. Calcipotriol ointment 50 micrograms/g twice daily was applied to one-half of the body. On the opposite side, white soft paraffin was applied in the morning, and coal tar solution BP 15% v/w in aqueous cream in the evening. Thirty patients with stable chronic plaque-type psoriasis were recruited. Assessments were made at 2, 4 and 6 weeks. Three patients were withdrawn from the study. A decrease in PASI score was seen on both sides at 2, 4 and 6 weeks. The differences from baseline between the two treatments were statistically significant in favour of calcipotriol. Improvement with calcipotriol was rapid in the first 2 weeks of treatment. With tar, significant improvement occurred only after 4 weeks of treatment. The differences in the scores for erythema, induration and desquamation from baseline between the two treatments were also statistically significantly in favour of calcipotriol at all evaluation points. Seven patients developed irritation on the calcipotriol-treated side, but there were no adverse effects on the tar-treated side. In two patients, itching associated with psoriasis was reduced by the calcipotriol. Although the mean serum calcium and phosphate levels remained within the normal ranges after 6 weeks' treatment, there were significant changes in their values compared with baseline.