Short-contact treatment with dithranol (anthralin) is a widely used treatment for chronic plaque psoriasis. Although effective, it causes staining and irritation, and is therefore inconvenient. Calcipotriol is a recently developed vitamin D analogue which is effective and easy to use. To evaluate the relative efficacy, safety and acceptability of these treatments a multicentre, open, randomized, parallel-group comparison was performed. Four hundred and seventy-eight patients with chronic plaque psoriasis were randomized to use one of the two treatments for 8 weeks. One group applied calcipotriol ointment (50 micrograms/g) twice daily. The other used a single application for 30 min each day of Dithrocream in the highest concentration tolerated. Severity of psoriasis was assessed by modified PASI score at baseline, and after 2, 4, and 8 weeks of treatment. A five-point scale was used by subjects and by investigators as an additional assessment of overall response, and a similar scale was used by subjects to grade acceptability. Total serum calcium was monitored at baseline and after 2 and 8 weeks on treatment. The mean PASI score fell from 9.1 to 4.7 after 8 weeks on dithranol (P < 0.001), and from 9.4 to 3.4 on calcipotriol (P < 0.001). The difference between the two treatments was significant in favour of calcipotriol at 2 weeks (P < 0.001), and remained so at subsequent assessments. At 8 weeks the difference between mean improvements in scores for the two groups was 1.6 (95% confidence interval 0.5-2.7). Efficacy grading by subjects and investigators, and acceptability grading by subjects, were all significantly better for calcipotriol.(ABSTRACT TRUNCATED AT 250 WORDS)