Intimal thickening is a significant cause of late failure of aorto-coronary vein grafts. The microscopic appearance of this thickening has some similarities to the microscopic appearance of arterial atherosclerosis, and it has been suggested that hyperlipidemia may play a role in its pathogenesis. This study examines the morphology and lipid composition of autologous vein and artery grafts in normal and hyperlipidemic rhesus monkeys. Grafts were examined six months after insertion by light and electron microscopy and tissue lipids were determined quantitatively. Intimal thickening occurred in all grafts. Specific morphological and lipid compositional features of the grafts were influenced by the type of tissue used for grafting and the presence or absence of hyperlipidemia. However, the degree of intimal thickening per se could not be related to either of these two factors. It is concluded that surgical transplantation in this model provides the most powerful stimulus for intimal thickening and any additional effect on this process by hyperlipidemia is small.