Ultrasound Doppler data are routinely used in the clinical diagnosis of degree of stenosis in the renal artery. In this study the effect of fat layers on maximum velocity estimations was determined using both a Doppler string and flow phantom. Using an ATL HDI 3000 with two broadband transducers (C4-2 and L12-5), the maximum velocity was estimated for a range of velocities with and without the presence of a fat layer. The results indicate that the introduction of a fat layer has a significant effect on the overestimation of the maximum velocity when the string phantom is used. However, in the case of the flow phantom which is more physiologically representative of in vivo conditions, underestimation of the maximum velocity was observed; this may cause patients to be diagnosed as having a smaller stenosis than is actually present.