Anastomotic parts of polyester vascular prostheses implanted in thoracic aortae of 270 dogs were examined. Anastomotic false aneurysms were seen with 3 cases (1.2%), in which disruption did not occur, however, the grafts which have no aneurysm showed the following features, i.e., suture migration, degenerative changes of the host arterial wall sustaining a suture and dissection between the host arterial wall and a prosthesis. From the results of an analysis of the arrangement pattern of smooth muscle cells in neointima, it was clarified that a suture had been subjected to continuous tensile stress. So that, the host arterial wall sustaining a suture is clinched and pulled toward prosthesis by the suture at every pulsation during implantation. These results indicate that the features mentioned above are phenomena of the developing into anastomotic false aneurysms.