In a randomised, double-blind trial 60 patients with left ventricular dysfunction (ejection fraction less than 45%) but without clinical evidence of heart failure 1 week after Q wave myocardial infarction were given captopril 25 mg thrice a day, frusemide 40 mg daily, or placebo. Left ventricular volumes were measured at baseline and at 1, 3, 6, 9, and 12 months with cross-sectional echocardiography and Simpson's rule analysis of standardised apical views. The captopril group showed no significant change in left ventricular end-diastolic volume index but left ventricular end-systolic volume index was significantly reduced and stroke volume index and ejection fraction were significantly increased from 1 month on. In contrast, the frusemide and placebo groups showed significant increases in ventricular volumes, with stroke volume index unchanged and ejection fraction slightly reduced. The changes in the captopril group were significantly different from those in the other groups.