The left ventricle progressively dilates in some patients after acute myocardial infarction (AMI). Both systolic and diastolic left ventricular (LV) dysfunction can be of significance in the development of heart failure. Captopril has been shown to prevent dilatation, but the effect on LV diastolic function is unknown. In a placebo-controlled double-blind parallel study, 58 AMI patients with heart failure or low ejection fraction, or both, were consecutively randomized at day 7 to either placebo or captopril (25 mg twice daily). No differences were present between the groups at baseline. Fifty-three patients completed the 6-month study period. Both LV diastolic and systolic volume indexes increased significantly in the placebo group (17 and 14%, respectively); in the captopril group there was no change in LV diastolic volume index, but a 13% reduction in LV systolic volume index. Ejection fraction increased significantly in the captopril group. The peak flow velocities of the early and atrial filling phases were measured, and the ratio between the velocities was calculated. A significant reduction was observed during the study period in early peak flow velocity (65 to 52 cm/s) and in the ratio between early and atrial peak flow velocity (1.3 to 0.8) in the placebo group (p less than 0.05), but no significant changes occurred in the captopril group. No correlation was found between dilatation of the left ventricle and reduction in early peak flow velocity or the ratio between early and atrial peak flow velocity. In conclusion, captopril prevented LV dilatation, improved ejection fraction and prevented LV diastolic dysfunction in AMI patients with early signs of LV systolic dysfunction.