Acute myocardial infarction (AMI) leads to left ventricular dysfunction, the extent of which predicts mortality. We studied the effect of very early enalapril treatment in patients with left ventricular failure (Killip classification II-III) resulting from AMI. In a double-blind randomized trial, patients on conventional treatment were started on placebo (PL, n = 15) or 2.5 mg enalapril (EN, n = 15) twice daily as early as 24 to 30 h after AMI and were followed up over a period of 21 days. One patient died in each treatment group. There were three dropouts in the placebo group (progressive heart failure requiring antiotensin-converting enzyme inhibition) and one dropout in the enalapril group (malignant ventricular arrhythmias). Plasma atrial natriuretic peptide (ANP) and norepinephrine decreased similarly in both groups from elevated baseline concentrations. The patients with the highest baseline ANP levels died in both groups: EN: 579 fmol/ml (mean 65.3 +/- 34.4 fmol/ml), PL: 403 fmol/ml (mean 63.5 +/- 37.6 fmol/ml). Killip classification improved in 9 of 13 patients on enalapril but only in 5 of 11 patients on placebo. On echocardiography an increase in fractional shortening (FS) (3.2 +/- 7.5%, p < 0.05) was found with enalapril only. Patients on placebo required more diuretics, and plasma aldosterone increased threefold. Thus, very early enalapril treatment may help prevent left ventricular failure after AMI. Extremely high initial plasma ANP concentrations may predict an unfavorable outcome.