Radionuclide determination of left ventricular ejection fraction was performed at hospital discharge and one month later in 60 patients who had suffered acute myocardial infarction. At the first determination, the patients were randomized into two groups. In the first group, the cardiologist who cared for the patients was provided with the result of the determination of the ejection fraction whereas, in the second group, the result was withheld. At a 'blinded' evaluation two months after hospital discharge, 7 of those patients (24%) from the group where the cardiologist knew the ejection fraction and 11 of the patients (38%) in whom this result was withheld complained of angina pectoris on exertion (ns). The medication of the patients did not differ in the two groups. No significant difference was found in the values of the ejection fraction in patients with and without angina pectoris. In this controlled study, we were not able to document a clinical effect by routinely determinating left ventricular ejection fraction in patients with acute myocardial infarction in the treatment of angina pectoris.