Abstract: The creation of arteriovenous fistulas in renal dialysis patients is associated with various problems. By placing a valve which can control the blood flow between artery and vein, material improvements in renal dialysis patient care can be achieved. In a preferred embodiment, the valve is mounted on a ring located around an aperture in the side of an asymmetrically expanded stent set in the artery. Novel stents, valve structures and valve ring insertion arrangements are also described.