Abstract The effects of early intervention with metoprolol in patients with suspected or definite acute myocardial infarction (AMI) have been assessed in a randomized, double-blind, placebo-controlled international study. Patients aged 75 years or younger were eligible for entry if they presented to a coronary care unit within 24 hours of the onset of symptoms of an AMI. Exclusion criteria included current treatment with a β blocker or calcium-channel blocker, heart rate ≤65 beats/min, systolic blood pressure ≤105 mm Hg, contraindications and other administrative reasons. Treatment began with an intravenous loading dose (3 × 5 mg injections of metoprolol or placebo at 2-minute intervals) followed by an oral regime of 200-mg metoprolol daily or placebo. The study period was 15 days in addition to the day of randomization. The patients' clinical history and status at entry were documented. The following outcome variables were recorded: mortality, development of AMI, serum enzyme activity, electrocardiographic signs of AMI, late or recurrent AMI, arrhythmias, treatment of chest pain, concomitant treatment, adverse events and details of treatment with trial medication.