A prospective study of duplex Doppler ultrasound in 29 renal transplants was undertaken to determine how to interpret Doppler findings in the immediate postoperative period. The study included intraoperative pulsed Doppler recordings from grafts immediately following release of vessel clamps. Subsequent follow-up studies were performed in the immediate postoperative period. Intraoperative Doppler appearances in the group as a whole were normal; there was a significant deterioration in Doppler appearances in the first 24-48 h of the postoperative period. In the absence of rejection subsequent Doppler appearances returned to normal. An abnormal Doppler appearance immediately following transplantation should be an expected result. If the Doppler fails to improve, or deteriorates having started to improve, rejection should be strongly suspected.