Treatment with angiotensin-converting enzyme inhibitors has a beneficial effect on myocardial ischemia and left ventricular dysfunction after myocardial infarction. The effect of captopril on myocardial ischemia was evaluated in 58 patients with left ventricular dysfunction (ejection fraction < 45%) after Q-wave or non-Q-wave myocardial infarction in a placebo-controlled, parallel, double-blind study. Patients were randomized on day 7 to either placebo or captopril (50 mg daily) and monitoring for a period of 180 days by serial echocardiography and ambulatory ST-segment monitoring. There was a significant effect of captopril on the duration of ambulatory ST depression during the 180 days: The values per day were reduced from 28 +/- 5 min at baseline to 2 +/- 1 min on day 180 in the Q-wave group (p < 0.01) and from 39 +/- 10 min at baseline to 6 +/- 1 min on day 180 in the non-Q-wave group (p < 0.05). In the placebo group the duration of ST depression on day 180 were 21 +/- 8 min in the Q-wave group and 22 +/- 7 min in the non-Q-wave group, thus being significantly higher as compared with the corresponding captopril groups (p < 0.01 and p < 0.05, respectively). In the placebo Q-wave group there was a significant increase in left ventricular end-diastolic volume index from 74 +/- 3.5 to 89 +/- 4.5 ml/m2 (p < 0.01) during the study period, which was in contrast to unchanged values of 75.5 +/- 3.0 and 75.0 +/- 3.5 ml/m2 (not significant [NS]) in the captopril Q-wave group.(ABSTRACT TRUNCATED AT 250 WORDS)