Ultrasonic examination of renal transplants with special attention to the parenchymal echo pattern has been undertaken in 43 patients. In the normal renal transplant, the pyramids, cortex and renal sinus can be distinguished. Renal transplant rejection is manifested by swelling and decreased echogenicity of the pyramids and hyperechogenic cortex. In addition, large anechoic areas due to hemorrhagic infarcts and necrosis are seen. In long-standing rejection, a normal or small-sized kidney with an irregular intrarenal echo pattern is observed. In 13 cases of acute tubular necrosis, none of the above appearances could be demonstrated. Serial ultrasonic scans are essential to reveal evolutionary changes of the rejection process.