The therapeutic efficacy and tolerability of calcipotriol ointment and betamethasone valerate ointment in psoriasis were compared in a multicentre, prospective, randomised, double-blind, right/left trial. 345 inpatients and outpatients with psoriasis vulgaris of symmetrical distribution were treated twice daily for 6 weeks with calcipotriol ointment 50 micrograms/g and betamethasone ointment 0.1% randomly assigned to opposite sides of the body. The main outcome measures--the psoriasis area and severity index (PASI), the investigators' assessments of erythema, thickness, and scaling, and the patients' own assessments of the overall response to treatment--were sought at weeks 2, 4, and 6. Both treatments significantly reduced the PASI scores and the investigator's assessment scores, but at each visit the PASI score was significantly (p less than 0.001) lower with calcipotriol than with betamethasone. At 6 weeks the mean PASI reduction was 68.8% with calcipotriol and 61.4% with betamethasone (95% confidence interval for difference 5.1-9.8, p less than 0.001). The scores for erythema, thickness, and scaling were significantly (p less than 0.001) lower with calcipotriol than with betamethasone at the end of treatment. The patients considered that 82.1% of calcipotriol-treated sides and 69.3% of betamethasone-treated sides had improved greatly or cleared up by the end of treatment (p less than 0.001). 57 adverse events were reported by 52 patients (15.1%). The most common adverse event, lesional/perilesional skin irritation, was slightly but not significantly (p = 0.12) more common with calcipotriol treatment. 15 (4.3%) patients were withdrawn from the study, 3 because of local adverse events. There were no changes in serum calcium during the study. Thus, calcipotriol ointment was superior to betamethasone valerate ointment in psoriasis vulgaris. Though long-term results are not yet available, calcipotriol holds great promise as an antipsoriatic agent.