The prophylactic effect of flunarizine and metoprolol was studied in a multi-center randomized, double-blind trial of 149 patients with migraine with or without aura. After a 4-week placebo run-in period, patients were randomly allocated to treatment with flunarizine 10 mg daily or metoprolol 200 mg daily for 16 weeks (parallel group design). Both drugs reduced the number of migraine days per month by 37% (95% confidence interval 21-53%) compared with the placebo run-in period. All efficacy parameters were significantly reduced by both drugs and no significant difference was found between the two drugs at any time of the treatment period. However, calculation of the 95% confidence limits showed that each drug may have a superiority of more than 100% on a single main effect parameter. The most common adverse experiences were day-time sedation (both drugs) and weight gain (flunarizine). Depression was the most serious side-effect occurring in 8% on flunarizine and 3% on metoprolol. We conclude that both drugs are effective in the prevention of migraine attacks but a higher number of dropouts occurred on flunarizine because of depression or weight gain.