Thrombogenicity of graft material, low velocity of blood flow, and wall collapsibility have been cited as the main factors responsible for the high occlusion rate of vascular prostheses placed in the venous system. This study was performed to analyze the effects of measures taken to overcome each of these factors. The thrombogenicity of expanded polytetrafluoroethylene (ePTFE) was evaluated by determination of the 3-hour deposition of radionuclide-labeled platelets and fibrin on grafts placed in the infrarenal vena cava of 18 dogs. Wetting the prosthesis with heparin before implantation significantly decreased platelet deposition at the cranial anastomosis (p less than 0.025) and on the graft surface (p less than 0.01), whereas the decrease of fibrin deposition was not statistically significant. The effects of flow velocity and graft support were studied in 44 dogs subjected to iliocaval bypass. The results of these studies demonstrated that a flow-increasing arteriovenous fistula was necessary to maintain patency of ePTFE grafts (p less than 0.01) but did not enhance patency of autogenous vein grafts. External ring support of ePTFE grafts did not significantly improve early patency. Decreasing thrombogenicity by wetting the grafts with heparin and increasing the blood flow by constructing an arteriovenous fistula helps in overcoming failure of venous vascular prostheses.