The purpose of this study was to investigate the patency rates and the histologic findings of neoendothelialization according to the diameter of polytetrafluoroethylene (PTFE) which is the most widely used synthetic graft for blood vessel. Under the operating microscope, grafts of 3 and 5mm in diameter were implanted in both femoral arteries of ten mongrel dogs by interrupted end-to-end microanastomosis. They were divided into two groups according to the diameter of implanted PTFE (3 and 5mm). Each group had five dogs. The lengths of implanted PTFE were 20mm in both groups. All implanted grafts were 25 urn in fibril length and 0.39mm in wall thickness. Two grafts from one dog were harvested at 1, 2, 4, 8, and 12 weeks after implantation and all PTFE grafts were observed for patency rates. The histologic analysis was performed under a light microscope and scanning electron microscope. In conclusion, PTFE with larger diameter than that of recipient vessel is recommended as the grafts for implantation because PTFE is not as elastic as normal vessel.