Stenosis of at least 50% of both the left anterior descending and circumflex coronary arteries proximal to their major branches has sometimes been called left main equivalent (LME) disease. During a 4-year interval, 189 LME patients were identified as surgical candidates by angiographic criteria. Coronary artery bypass grafting (CABG) was performed in 119 patients; 70 patients were managed nonoperatively. Treatment assignment was nonrandom. During the same interval, 203 patients with significant left main coronary artery (LMCA) stenosis and 742 patients with three-vessel disease without LMCA or LME stenosis underwent CABG. Follow-up is 99.2% complete at an average of 48 months (range 24--73 months). Operative mortality was significantly higher among LMCA patients (4.0%) than among LME surgical patients (0.8%) (p less than 0.05). Cumulative 5-year survival was better among LME surgical patients (98.2 +/- 1.3%) than among LME medical patients (75.9 +/- 5.7%) (p less than 0.0001) or LMCA surgical patients (92.8 +/- 2.1%) (p less than 0.03). All operative patient groups had a significantly lower incidence of myocardial infarction (10.3--13.4%) than the LME medical patients (25.7%) (p less than 0.05). Relief of angina was also significantly better in the operative groups (61--67.6%) than in LME medical patients (36%) (p less than 0.005). Major differences exist between LME and LMCA patients; LME is more properly considered a variant of multivessel coronary artery disease. Results of medical and surgical therapy suggest that patients with LME disease should be strongly considered for early operative therapy.