In the 54 patients studied coronary arteriography alone showed stenosis of the left main coronary artery (a lesion observed in 5.2 p. 100 of pathological coronary arteriographies), associated with other coronary lesions, 72.2 p. 100 of which involved all three major vessels. Ventriculography was abnormal in 53 p. 100 of the cases, with a less than 45 p. 100 left ventricular ejection fraction in 37.4 p. 100 of them. Nine patients were considered inoperable; 4 of these died during a 40-month follow-up period under medical treatment. Forty-five patients underwent coronary bypass; there were 4 peri-operative deaths (8.9 p. 100), 8 post-operative myocardial infarctions (17.8 p. 100) and one late death (2.2 p. 100). These patients were followed up for a mean period of 26.4 months. All authors who have studied the subject agree that surgery is beneficial: not only does it improve cardiac function, but it also prolongs survival. In spite of the peri-operative risks, surgery must be contemplated in patients with a more than 50 p. 100 stenosis of the left main coronary artery, provided the cardiac vascular bed is accessible to revascularization.