Left ventricular dysfunction can be improved with angiotensin-converting-enzyme inhibition started 1 week after myocardial infarction or later. To see whether earlier intervention may confer greater benefit, a double-blind study was carried out in which 100 patients with Q wave myocardial infarction, but without clinical heart failure, were randomly allocated treatment with captopril 50 mg twice daily or placebo starting 24-48 h after onset of symptoms. Left ventricular volumes were measured regularly during 3 months of treatment and after a 48 h withdrawal period by means of two-dimensional echocardiography. The placebo group showed significant increases in left ventricular end-diastolic (LVEDVI) and end-systolic (LVESVI) volume indices, with the ejection fraction unchanged. By contrast, the captopril group showed a slight but not significant rise in LVEDVI and a significant reduction in LVESVI with ejection fraction increased significantly. At 3 months there was a 4.6% difference in the change in ejection fraction from baseline between the groups (p less than 0.0001). Most of the treatment benefit was evident at 1 month and there were no changes in left ventricular volumes after 48 h withdrawal of treatment at 3 months. Heart failure requiring treatment with frusemide developed in 7 patients in each group during the study period; 3 of these (1 captopril-treated, 2 placebo-treated) had to be withdrawn from the trial with severe heart failure requiring open treatment. Thus early treatment with captopril is effective in preventing the ventricular dilatation that can occur after Q wave myocardial infarction.