Aneurysmal dilatations in saphenous vein grafts are rare complications of coronary artery bypass surgery that mostly represent thin-wall pseudoaneurysms at anastomotic sites. We describe a case of an enlarging distal saphenous vein graft aneurysm in which intravascular ultrasound (IVUS) and computerized three-dimensional reconstruction (3DR) of the IVUS images was performed to conclusively demonstrate true aneurysm morphology. Although both atherosclerotic and nonatherosclerotic mechanisms for vein graft aneurysm formation have been previously suggested, IVUS images and 3DR of the aneurysm in this case did not reveal any of the features typical for atherosclerotic lesions. Further, the IVUS images and 3DR suggest that progressive atherosclerosis is not the likely cause of aneurysm formation in this case. This application of IVUS and 3DR provides detailed information about saphenous vein graft aneurysm structure, clues to aneurysm formation, and suggests a natural history that may differ from that of pseudoaneurysms.