Renal transplants: can acute rejection and acute tubular necrosis be differentiated with MR imaging? Magnetic resonance (MR) imaging was used in 40 renal transplant recipients to determine whether this modality can enable distinction of acute tubular necrosis (ATN) and acute rejection by means of corticomedullary differentiation (CMD). Each patient underwent initial MR imaging after allograft renal transplantation. Twenty-nine of these 40 patients (72%) also underwent subsequent follow-up MR imaging. Seventeen studies were obtained during episodes of ATN; 12 of these studies (71%) showed poor CMD. Eleven studies were obtained during episodes of acute rejection; eight of these studies (73%) showed poor CMD. In addition, six of seven studies (86%) showing various combinations of renal disease (ATN, acute rejection, chronic rejection, and cyclosporine toxicity) also showed poor CMD. Loss of CMD is reversible after improvement of ATN and acute rejection. Because loss of CMD is a nonspecific though sensitive sign reflecting renal transplant dysfunction, MR imaging is of limited value in the differentiation of ATN from acute rejection. 