Aortocoronary bypass surgery has been performed most often with the patient's saphenous vein as the conduit. The internal-thoracic-artery graft, which has superior patency rates, has been shown to have clinical advantages, but it is not known how long these advantages persist. We identified all the patients in the registry of the Coronary Artery Surgery Study who had undergone first-time coronary-artery bypass grafting. Those with internal-thoracic-artery bypass grafts (749 patients) were compared with those with saphenous-vein bypass grafts only (4888 patients) with respect to survival over a 15-year follow-up period. In a multivariate analysis to account for differences between the two groups, the presence of an internal-thoracic-artery graft was an independent predictor of improved survival and was associated with a relative risk of dying of 0.73 (95 percent confidence interval, 0.64 to 0.83). This improved survival was also observed in subgroups including patients 65 years of age or older, both men and women, and patients with impaired ventricular function. The survival curves of the two groups showed further separation over the years of follow-up, with a more marked downsloping after eight years in the curve for the group with saphenous-vein grafts only than in that for the group with internal-thoracic-artery grafts. As compared with saphenous-vein coronary bypass grafts, internal-thoracic-artery grafts conferred a survival advantage throughout a 15-year follow-up period. The survival advantage increased with time, suggesting that the initial selection of the conduit was a more important factor in survival than problems appearing long after surgery, such as the progression of coronary disease.