The reliability of ultrasonography in detecting complications of renal transplantation was investigated by reviewing the ultrasound examination on 78 renal transplants. The results of the ultrasound examinations were correlated with all available clinical, laboratory, radiographic, scintigraphic, and histopathologic data. With the exception of one urine leak no extrarenal complications of clinical significance, such as hydronephrosis, urine leak, lymphocele or hematoma were overlooked at ultrasonography. The exact nature of fluid collections was, however, only seldom accurately assessed unless fine needle biopsy was used. The accuracy in detecting both acute and chronic rejection was high with ultrasonography and a normal ultrasonography of a non-functioning kidney was a strong indication of acute tubular necrosis. Ultrasonography was very sensitive in detecting early acute rejection but was of less value in diagnosing repeated acute rejections with short intervals and in monitoring the response of treatment as the sonographic changes of rejection disappeared slowly.