Intimal thickening of uncertain cause has been noted in saphenous vein bypass grafts. To study the development of these changes, 97 vein grafts in 55 autopsied patients were examined by angiography and histology. Significant alterations in the grafts were confined to the intima and included thin layers of circumferential thrombus, concentric fibrous plaque and occlusive thrombus with organization. Circumferential nonocclusive intimal thrombus was present in 36 (73%) postoperative patent grafts. Concentric fibrous plaque, found in all twelve patent late grafts, showed changes characteristic of atheromata, including smooth muscle and foam cells. Intimal fibroelastosis was absent in the grafts, indicating that concentric plaque is not a simple response to increased pressure, or an "arterialization" of the vein. The findings suggest that early appearing concentric intimal thrombus is converted into fibrous plaque, and that this early thrombus is a factor in the development of acclereated "atherosclerosis" in saphenous vein bypass grafts.