Expanded polytetrafluoroethylene (ePTFE) external valve support devices (EVS) have been used successfully in patients to restore valve function in leg veins with incompetent valves when incompetence is due to dilatation of the vein walls or elongation of the valve leaflet edges. To assess tissue response to these devices, the authors implanted 12 of them in dogs, wrapping the devices around veins in the head and neck. The dogs recovered from the implantation procedure uneventfully, and the veins remained patent on color flow Doppler scanning. Gross and histologic evaluations of vein segments and attached EVS devices after sacrifice of the dogs 30 days postoperatively showed that the ePTFE devices did not affect vein patency or the cellular composition or architecture of vein walls. There were no adverse tissue reactions to the EVS and no thrombus formation in the veins to which the EVS had been applied. Tissue attachment to the EVS was apparent in all specimens. These histologic results support clinical experiences indicating that the ePTFE EVS device is safe to use in external valvuloplasty for the treatment of venous incompetence.