Of 4,196 patients undergoing coronary angiography, 297 (7 percent) had left main coronary arterial narrowing. In 188 (4.5 percent) the narrowing was greater than or equal to 70 percent and in 109 (3 percent) it was between 50 and 69 percent. Three patients (1 percent) died at cardiac catheterization. Saphenous vein bypass graft surgery was performed in 267 patients with an operative mortality of 7 percent: in 179 patients the left main coronary narrowing was greater than or equal to 70 percent (operative mortality 9 percent), and in 88 between 50 and 69 percent (mortality rate 2 percent). There was an average of 2.6 grafts per patient. The course of these patients was followed up for 4 3/4 years. At 1 year there was a 92.2 percent survival rate. The rate of survival at 2, 3 and 4 3/4 years, was 91, 89 and 86 percent, respectively. After saphenous vein bypass graft surgery, 75 percent of patients are angina-free and 22.4 percent are in improved condition. Grafts were studied in 51 patients and 85 percent of 128 grafts were found to be patent. It appears that there is improvement in survival and a reduction of symptoms after saphenous vein bypass graft surgery in patients with left main coronary arterial narrowing.