Of 686 male adults randomly allocated to 2 treatment groups in the 3 years 1972, 1973, arid 1974, there were 354 assigned to the medical (control) group and 332 assigned to medical plus surgical treatment (in 95% of cases, to saphenous vein aortocoronary bypass procedure alone). There were no statistically significant differences between the 2 groups in any of 35 different clinical, electrocardiographic and arteriographic characteristics at baseline, except for serum cholesterol, so the 2 groups were judged to be comparable. Twelve percent of patients were found to have significant disease of the left main coronary artery, in almost all instances in addition to disease of other vessels. For this subgroup, surgery was associated with increased survival (at 4 years, p < 0.001). For the remaining 88% of patients, and for the 6 subgroups into which they could be categorized on the basis of extent of disease and status of left ventricular function, no significant differences in survival could as yet be identified in these preliminary analyses. Another important finding was that inclusion of patients with left main coronary artery disease in the analyses of subgroups with 3-vessel disease gave results for survival favoring surgically randomized patients, to a statistically significant degree. If patients with left main coronary artery disease were excluded, the difference in survival between medically and surgically treated patients was no longer statistically significant. Medically treated patients had better survival rates than earlier reports in the literature had indicated.