In 222 patients, 741 venous coronary bypass grafts were studied angiographically early, at 1 year and at a late examination at greater than 6.5 years (mean 9.6) after operation; 565 of these grafts were also examined 5 years postoperatively. Grafts were graded for patency and disease considered to be atherosclerotic and for both extent and profile of lesions. Graft occlusion rates increased steadily from 8% early to 20% at 5, 41% at 10 and 45% at greater than 11.5 years after operation. All grafts were considered free of atherosclerosis early, but disease appeared in 8% at 1 year, increasing to 38% at 5 and 75% at 10 years postoperatively. Increasing involvement of vessel wall area was associated with greater protrusion of lesions into the graft lumen. Diseased grafts became more so at subsequent examinations, with occlusion occurring in many. However, absence of disease had little prognostic significance because diseased and abruptly occluded grafts were generated in those with healthy appearance at earlier examinations. For instance, 82% of very diseased grafts at the 5 year study originated from normal grafts at 1 year and 73% of occluded grafts at 1 year had appeared normal early postoperatively. Of 590 patent grafts free of disease at 1 year, 30% were occluded at the late examination, 76% of those patent were diseased, 55% of these were diffusely diseased and 35% were greater than 50% narrowed. Only 17% of the original 590 patent grafts were healthy at this time. Bypass graft atherosclerosis severely limits the long-term utility of these grafts. It is suggested that the solution may lie in some powerful drug regimen.