OBJECTIVE It is known from experiments that angiotensin converting enzyme (ACE) inhibitors can limit infarct size. We examined the effect in patients. METHODS In a prospective, randomized, placebo-controlled double blind study, 22 patients were given 1.5-2.0 mg captopril/h i.v., while 24 patients were given placebo. Medication was started between 2 hours and 18 hours from the onset of infarction. The two groups were matched for age, infarct location, and time of intervention. With exception of one patient in either group, all were concurrently given nitroglycerin. The necrosis parameters were provided by the quantitative measurement of the QRS complex. RESULTS The Q wave decreased with captopril treatment (-0.003 mV), but increased with placebo (+0.14 mV) (p < 0.05). The number of ventricular premature beats at 24 hours from the start of treatment was 25/h with placebo, and 9/h with captopril (p < 0.02). Ventricular fibrillation occurred 7 times in the placebo group, but did not occur in the captopril group. The creatine kinase (CK) infarct weight was 59 gram-equivalents (gEq) with placebo, and 45 gEq with captopril (p = NS). The mean arterial pressure was reduced by 12 mmHg with captopril treatment. CONCLUSIONS The results show a beneficial effect of captopril on infarct size and electrical instability, over and above the effect of standard management with nitroglycerin and thrombolysis.