Factors related to early and late mortality were studied in 663 consecutive patients who had coronary bypass operations. There were 18 operative deaths (2.7%) and 15 late deaths (2.3%). Patients who died were slightly older than surviving patients. Operative mortality was higher (5.3%) in those with congestive heart failure; this correlated directly with the degree of abnormal wall motion shown on left ventriculograms. Preoperative end-diastolic pressure of the left ventricle was not a good predictor of death. While operative mortality was 1.5% in those with stable angina, it increased to 4.2% in patients with unstable angina and to 25% in patients with evolving myocardial infarction. In those with stenosis of the left main coronary artery, early mortality was 12.3%. Although early mortality was unaffected by the extent of coronary disease or by the degree of correction, it increased significantly (P less than 0.05) with the number of grafts inserted and when other cardiac procedures were also performed. Perioperative myocardial infarction was associated with a 28% mortality, but was unrelated to graft failure in 60% of the cases. Late mortality was related only to the number of diseased coronary arteries. Thus, left ventricular function, severity of angina and extent of coronary obstruction appear to be the main determinants of survival following coronary artery operation.