INTEREST in the management of patients with coronary artery disease has focused on subgroups of patients defined by arteriographic findings. In the Veterans Administration Cooperative Study, the first subgroup with significantly improved survival when managed surgically and followed for 2-3 years was that of patients with disease of the left main coronary artery.1 Other reports, including the only other randomized controlled study,2 confirmed this finding.Patients with left main disease are not a homogeneous group.3'4 Those with more severe stenosis or with an abnormality of left ventricular (LV) function were reported to have a worse prognosis.4 In coronary heart disease patients without left main disease, a benefit from surgery has also been reported in subgroups based on clinical criteria alone.5 In this report, we examine the effects of bypass surgery on survival and on the incidence of myocardial infarction (MI) in angiographically and clinically defined subgroups of patients with left main coronary artery disease. tients who had a significant stenosis (at least 50% reduction of the luminal diameter) of the left main coronary artery. The coronary arteriograms were reviewed to confirm the presence of a significant left main lesion and to assess the degree of stenosis. MIs that occurred after randomization were identified in accordance with definitions used by the Coronary Drug Project8 and by Hultgren et al.9 (appendix A). ECGs and autopsy protocols, as well as most left ventriculograms, were reviewed centrally (appendix B).In