Stenotic intimal thickening at the venous end of prosthetic arteriovenous (AV) fistulas for hemodialysis has been associated with perianastomotic mismatch in elastic properties between prosthesis and vein and high flow velocities. In a prospective study, we investigated the role of flow velocity on the occurrence of intimal hyperplasia in prosthetic AV fistulas in hemodialysis patients. In 24 patients, the flow velocities were assessed in the distal graft and the outflow vein postoperatively, with the use of vessel wall Doppler tracking, a noninvasive ultrasound technique. The initial velocity in the venous anastomoses was correlated with the occurrence of stenoses during follow-up (2 years). The detection of a stenosis was performed with both Duplex ultrasound and angiography. In 4 cases a stenosis developed in the venous anastomosis, in 8 cases in the venous outflow segment, and in 4 cases at both sites. Higher flow velocities around the venous anastomosis was observed in fistulas developing a stenosis at this site as compared with the nonstenotic fistulas (p < 0.05). The occurrence of stenoses in prosthetic AV fistulas in or adjacent to the venous anastomosis is not associated with a mismatch in elastic properties, but with high flow velocity.