In the past three years, among 170 patients undergoing aortocoronary bypass surgery, 11 (6%) developed acute myocardial infarction within 24 hours after surgery. An additional four patients (2%) developed myocardial infarction within three months after discharge. Clinically, acute myocardial infarction was suspected because of sudden, transient hypotension associated with dysrhythmia, angina, or cardiac arrest which responded to conventional therapy. Elevation of serum enzymes with acute ECG changes was also observed. Three of the 15 patients developing myocardial infarction died. In 12 patients cardiac catheterization studies were performed within two to ten weeks after the incident. Eleven of the 20 grafts were found occluded, and progression of coronary occlusive disease was seen in five. There was a marked decrease in left ventricular function, contractility, and compliance in all patients with left ventricular aneurysm formation or dyskinesia. Eight of these patients were asymptomatic. The results indicate that after coronary surgery a combination of sudden arrhythmia and transient hypotension is diagnostic of graft closure or development of acute myocardial infarction. Also, in spite of depressed cardiac function, most surviving patients remain angina free.