The authors retrospectively investigated the utility of the resistive index (RI) in evaluating the major causes of renal allograft dysfunction. Three hundred fourteen duplex US studies in 162 patients with 150 episodes of renal allograft dysfunction within a 17-month period were reviewed. Histologic findings were available in 69 cases. Three patients had hyperacute rejection with a mean RI of 0.85 ± 0.049. There were 37 episodes of acute rejection (mean RI, 0.76 ± 0.054) and 22 instances of chronic rejection (mean RI, 0.71 ± 0.065). Ureteral obstruction (nine cases; mean RI, 0.72 ± 0.026) was the only cause of allograft dysfunction other than rejection with a mean RI greater than 0.70. Mean RI values associated with rejection were significantly elevated above those in the 88 cases of dysfunction without rejection (mean, 0.64 ± 0.064) and in baseline examinations (mean, 0.63 ± 0.066). The results identified two causes of increased RI values in addition to acute rejection: chronic rejection and ureteral obstruction.