OBJECTIVE The purpose of this study was to assess the impact of arteriovenous fistulas combined with varying degrees of stenosis on distal bypass hemodynamics and Doppler spectral parameters. METHODS In an in vitro flow model bypass stenoses causing 30%, 55%, and 70% diameter reduction were induced 10 cm upstream of a fistula with low outflow resistance. Flow and intraluminal pressure were measured proximal to the stenosis and downstream of the fistula. The waveform parameters peak systolic velocity, end-diastolic velocity, pulsatility index, and pulse rise time were determined from midstream Doppler spectra obtained 10 cm downstream of the fistula. All measurements were carried out with open and clamped fistula. RESULTS At 30% diameter reducing stenosis opening of the fistula induced a 12% systolic pressure drop across the stenosis but had no adverse effect on the Doppler waveform parameters. At 55% stenosis the pressure drop increased from 16% to 31% after fistula opening. This increased pressure drop was associated with a further reduction in peak systolic velocity, a decrease in pulsatility index, and an enhanced pulse rise time prolongation. Fistula opening at 70% stenosis increased the systolic pressure drop from 31% to 48% and had significant impact on all waveform parameters. CONCLUSIONS Distal arteriovenous fistulas enhance pressure loss across stenoses and affect downstream velocity waveform configuration. The presence of a combined fistula and a stenosis mimics the distal hemodynamic conditions of a more severe stenosis. Assessment of the hemodynamic impact of fistulas must be undertaken in the evaluation of in situ vein bypass stenoses.