To clarify the process of graft degeneration and aneurysm formation we reviewed the angiographic and pathologic findings of three cases where aneurysms developed in glutaraldehyde-tanned human umbilical vein grafts. Seven aneurysms were detected in these three grafts 4 years after implantation. Six aneurysms originated from the bodies of the grafts and one originated from the factory-created anastomosis required for fabrication. Wrinkling and segmentation of the dilated graft documented by arteriography corresponded to mural dissection of the graft wall. Large protrusions near the aneurysms were revealed to be the transmurally dissected graft walls. At the site of dissection, blood escaped through the surrounding mesh of the graft to form a preaneurysmal change. Two aneurysms developed without breakdown of the outer mesh. The aneurysm of factory-created sutures was caused by tearing of the graft wall. Degeneration of human umbilical vein grafts, which appears to ensue inevitably with the passage of time after implantation, leads to the tear of the graft wall and transmural dissection to form aneurysm. In our department, candidates for human umbilical vein graft implantation have recently been restricted to the patients whose life expectancy is limited to only a few years because of the risk of aneurysm formation.