ONe hundred twelve patients undergoing aortocoronary bypass--35 with diabetes of adult onset and 77 without diabetes--were studied to determine whether diabetic patients have additional operative risks and greater operative mortality and whether their coronary disease differs from that of nondiabetic patients. Among the diabetic patients there was a greater prevalence of preoperative unstable angina, prior myocardial infarction and class IV functional disability (New York Heart Association criteria). The major coronary arteries angiographically and at operation appeared similar in both groups. The blood flow rates measured in aortocoronary bypass vein grafts were similar in both groups, raising doubt about the presence of microvascular disease in the myocardium of the diabetic patient. Preliminary follow-up results demonstrated relief of anginal symptoms in 76 percent of diabetic and 78 percent of nondiabetic patients. The operative mortality rate of 9 percent in diabetic and 4 percent in nondiabetic patients occurred among the first 40 patients in the series; no patient in either group has died in the immediate postoperative period during the last 18 months of the study. Aortocoronary bypass should be recommended to diabetic patients with symptomatic coronary arteriosclerosis using the same criteria for operability applied to the nondiabetic population.