Combined proximal left anterior descending and proximal left circumflex, or "left main equivalent" (LMEQ), disease defines a prognostic angiographic high-risk patient population. We assessed the effect of coronary bypass surgery compared with medical therapy in 903 patients with LMEQ disease by stratified life table and Cox regression analysis. The 5 year survival rates of the 639 and 264 patients who received surgical vs medical therapy was 85% vs 55%, respectively (p less than .001). Analysis of patient subsets stratified by age, angina class, right coronary disease, and ejection fraction revealed a significant survival benefit for surgically treated patients in most strata. Cox regression analysis revealed improved survival for surgically treated patients after adjustment for important baseline variables known to influence prognosis. Surgically treated patients had significantly less angina and need for antianginal drugs compared with the medically treated group. When the Coronary Artery Surgery Study randomized and randomizable LMEQ patients were analyzed, coronary bypass surgery improved 5 year survival when preoperative ejection fraction was under 0.50 but not when ejection fraction was 0.50 or higher. Thus coronary bypass surgery prolongs and improves quality of life (as defined by angina status and need for antianginal drugs) in most patients with LMEQ disease but does not appear to improve 5 year survival in a small subset of LMEQ patients who are asymptomatic after myocardial infarction or who have mild chronic stable angina and are under age 65 with well-preserved left ventricular function.