Certain clinical and necropsy findings are described in 53 patients who died from 13 to 185 months (mean 58) after a single aortocoronary bypass operation. Of the 53 patients, 32 (60%) died of a cardiac cause and of their 72 saphenous vein aortocoronary conduits, 36 (49%) were narrowed at some point more than 75% in cross-sectional area by atherosclerotic plaque; the remaining 21 patients (40%) died of a noncardiac cause and of their 50 saphenous vein conduits, 10 (20%) were narrowed at some point more than 75% in cross-sectional area by plaque. Thus the noncardiac mode of death in a large percentage of the patients suggests that the bypass operation prolonged life to a degree sufficient for another condition to develop. The 123 saphenous vein conduits were divided into 5 mm segments, and a histologic section was prepared from each. Of the 1104 five-millimeter segments in the 32 patients dying as a consequence of myocardial ischemia, 291 (26%) were narrowed more than 75% in cross-sectional area by plaque; in contrast, of the 761 five-millimeter segments of veins in the 21 patients with a noncardiac mode of death, 86 (11%) were narrowed more than 75% by plaque. Of the total 1865 five-millimeter segments of vein, only 395 (21%) were narrowed 25% or less in cross-sectional area by plaque. Thus, in patients dying late after coronary bypass the atherosclerotic process continues in all segments of the saphenous veins used as aortocoronary conduits. Therapy after the operation must be directed toward prevention of progression of the atherosclerosis in the "new" coronary "arteries."