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In order to evaluate survival patterns in patients with left main coronary artery (LMCA) disease, 1492 patients with LMCA disease admitted to the U. S. Collaborative Study on Coronary Artery Surgery (CASS) were followed up. Of the 1492 patients, 1183 (79%) underwent coronary artery bypass (CABG) with an operative mortality of 4.2% and an average of 2.7 grafts/patient inserted. Allocation to surgery was by patient and physician preference and not random. Surgical patients generally had more severe angina, a higher prevalence of beta blocker usage, worse ventricular function and more severe LMCA stenosis than medically treated patients. Overall (4 years) survival of the surgical group was 88% compared to 63% in the medical group (p less than 0.001). Other independent predictors of mortality included angiographic left ventricular (LV) dysfunction score, age, % LMCA stenosis, hypertension, dominance, and presence of significant right coronary artery (RCA) stenosis greater than or equal to 50%. CABG was not shown to significantly improve survival in women, in LMCA patients with left dominant circulation, in LMCA patients with non-diseases dominant or balanced RCA circulation or in LMCA patients with greater than or equal to 50% RCA stenosis but good LV function. These data indicate that CABG prolongs life in most patients with left main coronary artery disease, particularly those with severe LMCA narrowing or severe LV dysfunction, however, subgroups of LMCA patients are identifiable who may fare with medical treatment alone.