The effect of metoprolol on mortality and morbidity after 15 days, was compared with that of placebo in a double-blind randomised international trial (the MIAMI trial) in patients with definite or suspected acute myocardial infarction (AMI). Treatment with intravenous metoprolol (15 mg) or placebo was started shortly after the patient's arrival in hospital within 24 h of the onset of symptoms, and then oral treatment (200 mg daily) was continued for the study period (15 days). Of the 5778 patients included, 2901 were allocated to placebo and 2877 to metoprolol. Definite AMI was confirmed in 4127 patients. There were 142 deaths in the placebo group (4.9%) and 123 deaths in the metoprolol group (4.3%), a difference of 13 per cent with 95 per cent confidence limits of -8 to +33 per cent, not statistically significant (P = 0.29). Previously recorded risk indicators of mortality were analysed in retrospect. These indicated that there was a category which showed higher risk which contained approximately 30% of all randomized patients. In these, the mortality rate in the metoprolol treated group was 29% less than in the placebo group. In the remaining lower risk categories there was no difference between the treatment groups. This subset analysis must be interpreted with caution in view of the findings from other similar studies. Positive effects were observed on the incidence of definite AMI and on serum enzyme activity in patients treated early (less than 7 h). There was no significant effect on ventricular fibrillation but the number of episodes tended to be lower in the metoprolol treated patients during the later phase (6-15 days; 24 vs 54 episodes). The incidence of supraventricular tachyarrhythmias, the use of cardiac glycosides and other antiarrhythmics, and the need for pain-relieving treatment were significantly diminished by metoprolol amongst all randomised patients. Adverse events associated with metoprolol were infrequent, expected, and relatively mild.