The efficacy, tolerability and safety of calcipotriol solution and betamethasone 17-valerate solution were compared in a multicentre, prospective, randomized, double-blind, parallel group study. Four hundred and seventy-four patients with scalp psoriasis were recruited from six European countries and Canada. Following a 2-week washout period, either calcipotriol solution (50 micrograms/ml) or betamethasone 17-valerate solution (1 mg/ml) was applied twice daily for 4 weeks. After this time, patients who required no further active treatment were observed for relapse. Retreatment with calcipotriol was offered to those patients who relapsed, and who were originally in the calcipotriol-treated group. The two treatment groups were well matched at baseline. At the end of treatment, the proportion of patients who had 'cleared' or 'markedly improved' was statistically significantly greater in the betamethasone group (75%) than in the calcipotriol group (58%) (P < 0.001) (95% confidence interval of difference 25.3-->8.6). The decrease in total sign score (sum of scores for erythema, thickness and scaliness) at the end of treatment was also statistically significantly greater in the betamethasone group (61%) than the calcipotriol group (45%) (P < 0.001) (95% confidence interval of difference 9.7-->23.1). Adverse events were reported by 87 patients in the calcipotriol group, and 31 patients in the betamethasone group; the most common was lesional or perilesional irritation, which occurred significantly more frequently with calcipotriol (26%) than with betamethasone (8%) (P < 0.001). Fifteen patients (6%) in the calcipotriol group and four (1%) in the betamethasone group withdrew from the study because of adverse events or unacceptable treatment response (P = 0.017).(ABSTRACT TRUNCATED AT 250 WORDS)