Left main coronary artery disease (i.e., .50% stenosis) was found in 1,477 of 20,137 patients in the Coronary Artery Surgery Study (CASS) registry. Of these patients, 53 (3.6%) were asymptomatic. Asymptomatic and symptomatic patients were similar in regard to 1) severity of left main coronary artery stenosis (67% vs. 70%), 2) extent of proximal coronary artery disease (no differences in number of or severity of proximal stenoses), 3) left ventricular end-diastolic pressure (13 mm Hg vs. 14 mm Hg), 4) left ventricular wall motion score (9.1 vs. 8.7), and 5) number of coronary artery segments with greater than 70% stenosis (4.4 vs. 4.8). Among the asymptomatic patients, 47% received medical and 49% received surgical treatment. In the symptomatic group, 20% received medical and 78% received surgical therapy. The survival rate 5 years after surgery for treatment of left main coronary artery stenosis was 84% for the symptomatic patients and 88% for the asymptomatic patients (p=NS). Medical management of left main coronary artery disease produced a 5-year survival rate of 57% for asymptomatic patients and 58% for symptomatic patients. Within the asymptomatic subgroup, 88% of those surgically treated survived 5 years, whereas only 57% of those medically treated survived 5 years (p=0.02). Thus, for CASS patients with left main coronary artery disease, the percentage of those that were asymptomatic is low (3.6%); asymptomatic and symptomatic patients with left main coronary artery disease had no significant difference in severity of left main coronary artery stenosis, extent of overall coronary artery disease, or left ventricular function. Finally, in this nonrandomized series, survival in patients with asymptomatic left main coronary artery disease was significantly improved with surgery compared with medical management. (Circulation 1989;79:1171-1179 S everal randomized and observational studies show that the surgical treatment of symptomatic left main coronary artery disease improves survival significantly compared with medical management.1 17 Given the grim prognosis that left main disease has traditionally implied, these findings were not unexpected. However, close analysis of populations of patients with left main coronary artery disease suggests that the prognosis of From the Division of Cardiology