Total occlusion of the left main coronary artery was confirmed on review of the coronary angiograms in 12 (0.06 percent) of the 20,197 patients entered into the Coronary Artery Surgery Study (CASS) before coronary arterial surgery. Clinical features alone could not distinguish the patients with total occlusion of the left main coronary artery from those enrolled in the CASS with subtotal stenosis of this vessel. The right coronary artery had a stenosis greater than or equal to 70 percent of luminal diameter in 7 of the 12 patients. Collateral flow to the left coronary artery was defined as "substantial" or "limited" based on the presence or absence of clear visualization of the main channel of either the left anterior descending or left circumflex coronary artery during coronary angiography. Of the eight patients with "substantial" collateral flow, one (13 percent) had an aneurysmal or dyskinetic left ventricular wall segment, whereas all (100 percent) of the three patients with "limited" collateral flow had dyskinesia or an aneurysm (p < 0.05). Seven patients underwent coronary bypass graft surgery; 6 (86 percent) of these patients were living at their most recent follow-up, a mean of 46 months after entry into the CASS. Two of these patients continued to have angina pectoris. Five patients did not undergo coronary bypass grafting and 2 (40 percent) were still alive at their most recent follow-up, a mean of 45 months after entry into the CASS. One of these patients had angina pectoris. The difference in survival between the medical and surgical groups was not statistically significant. This study indicates that patients with total occlusion of the left main coronary artery are uncommon and cannot be distinguished by presenting features alone from patients having subtotal stenosis of the left main coronary artery. "Substantial" coronary collateral blood flow is associated with better left ventricular wall motion than is "limited" collateral flow. Prolonged survival and lessening of symptoms may occur after coronary bypass grafting although long-term survival is possible without it. © 1982.