To identify the factors associated with operative mortality, we evaluated clinical, angiographic, and surgical variables in 1172 patients with left main coronary-artery stenoses of at least 50 per cent, who underwent coronary-bypass procedures in the Collaborative Study in Coronary Artery Surgery (CASS). The operative mortality was 4.2 per cent overall and was less than 3 per cent in seven of the 15 participating hospitals. Historical variables associated with an increase in operative mortality were age, female sex, and duration and severity of angina. Other variables associated most closely with poor survival were urgency of operation, left coronary-artery dominance, severity of left main coronary-artery stenosis, and impairment of left ventricular contraction. The results of this multicenter study show that aortocoronary bypass surgery in patients with left main coronary-artery disease can be performed with a low mortality and that patients at high risk can often be identified before surgery.