Metoprolol slow-release tablets (Durules®), 200 mg, given once daily in the morning were compared with placebo in the prophylaxis of classic migraine. The trial comprised eight Scandinavian neurologic centres and was designed as a double-blind cross-over study with 4 weeks' run-in, four weeks washout, and 8 weeks of either treatment. Seventy-seven patients with two to eight migraine attacks per month were entered in the trial, and 73 completed it. A total of 1119 attacks with aura symptoms and 374 without were recorded. Metoprolol was significantly better than placebo with regard to the total frequency of attacks (1.8 versus 2.5 attacks/4 weeks), mean duration of attacks (6.0 versus 8.0 h/attack), mean global rating, and consumption of analgesics per attack: Similar differences could be shown for attacks with aura symptoms alone, except for the duration of attacks. Metoprolol is the first drug for which a prophylactic effect in classic migraine has been convincingly demonstrated.