Morphologic changes in saphenous vein-coronary artery bypass grafts in 24 [human] cases (51 grafts) 7-116 mo. after surgery were studied. Four pathologic changes were found: Fibrous cord lesions were present that represented early postoperative thrombotic occlusion and organization. Diffuse intimal thickening was present in all grafts maintaining long-term patency. Atherosclerosis identified as intimal foam cell accumulation of frank plaques was found only in cases 39 mo. or more postoperative and was present in 79% of such cases (71% of the vein grafts). Late vein thrombosis was found in 3 cases and was the presumed cause of death. Atherosclerosis apparently occurs with a high incidence of vein grafts after 3 yr, and plaque rupture with thrombosis can be a cause for vein occlusion.