We have conducted a randomized, double-blind, placebo-controlled multicentre trial of oral isosorbide 5-mononitrate (ISMN) in 360 patients with suspected acute myocardial infarction. Patients were stratified prior to analysis according to the presence or absence of left ventricular failure on admission. ISMN caused a significant reduction in systolic and diastolic blood pressure during the first 12 h. There was no significant effect on heart rate. Overall mortality was 4.9% in the ISMN group compared with 4.0% in controls at 5 days, and 14.1% compared with 10.5% at 6 months (NS). A non-significant reduction in mortality in the ISMN group with heart failure (ISMN 7.9%, placebo 12.9%, at 5 days) contrasted with a non-significant increase in mortality in patients without heart failure treated with ISMN (ISMN 4.1%, placebo 2.1%, at 5 days). Lignocaine was used in twice as many patients in the ISMN group as in placebo group (P less than 0.01), both with and without heart failure. Diamorphine usage was similar in the ISMN and control groups. Oral ISMN has similar haemodynamic effects to intravenous nitroglycerin, and can be of benefit in acute myocardial infarction with heart failure. However, our results question the use of nitrates in acute myocardial infarction in the absence of heart failure.