The "stunned" myocardium is the result of an ischemic insult which is not of sufficient severity to produce myocardial necrosis but affects myocardial function, biochemical processes and ultrastructure for a prolonged period of time. A "stunned" myocardium can be expected when preoperative asynergic wall motion is normalized after successful coronary bypass surgery under similar loading conditions. Forty-five patients (mean age 51 years) underwent diagnostic cardiac catheterization before and 9.5 months after successful bypass surgery. Patients with perioperative myocardial infarction and/or occluded vein grafts were excluded from the study. Preoperatively 17 of the 45 patients had a reduced systolic ejection fraction. These patients were further divided into the following two groups: group 1 consisted of 6 patients without or with only a small increase in regional ejection fraction (6 regions of the left ventricle in the right anterior oblique projection) after surgery (control group), and group 2 of 11 patients with a postoperative increase in preoperatively reduced regional ejection fraction (preoperatively "stunned" myocardium). In regard to duration of symptoms, functional classification (NYHA), number of myocardial infarctions, unstable angina pectoris and physical working capacity on the bicycle ergometer, there were no significant differences between the two groups. A mean 2.8 and 3.6 vein grafts (ns) were implanted in groups 1 and 2, respectively. Postoperatively, NYHA class decreased significantly and physical working capacity increased in both groups. Left ventricular end-diastolic and peak systolic pressure remained unchanged after surgery; left ventricular ejection fraction was unchanged in group 1 (49 vs. 51%, ns) but increased significantly in group 2 (38 vs. 50%, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)