The development of a reliable, minimally invasive method of distinguishing physiologically significant renal obstruction from dilation without obstruction would have important clinical implications. As it is well known that renal blood flow decreases over time in the presence of obstruction, we investigated the ability of color flow Doppler ultrasonography and dynamic contrast-enhanced CT scanning to detect changes in blood flow in unilaterally obstructed porcine kidneys. In the initial phase of this study, the effect of acute unilateral obstruction were studied in nine pigs. The resistive index (RI) was measured with Doppler ultrasonography, and renal blood flow was quantitated with dynamic CT using tracer kinetic principles and deconvolution. The RI measurements were unable to distinguish between the obstructed kidneys and their controls. Dynamic CT scanning demonstrated a greater fall in blood flow in the obstructed kidney, and this change was significantly different from baseline. The same findings were supported by radiolabelled microsphere blood-flow measurement. In the chronic portion of the study, after surgical creation of a partial ureteric obstruction, the kidneys were studied by both techniques at 1, 2, and 3 weeks. Again, RI was unable to demonstrate any difference between obstructed and unobstructed kidneys, while CT showed a progressive fall in blood flow in each successive week that was statistically significant. Dynamic contrast-enhanced CT scanning is a promising diagnostic tool that might be used to distinguish a functionally significant renal obstruction from nonobstructive dilation. Further clinical studies to validate this technique are warranted.