Renal transplant rejection is frequently difficult to differentiate from other causes of renal dysfunction. This study examined the use of duplex Doppler ultrasound and intrarenal manometry in a consecutive series of 73 patients who underwent renal transplantation. Altogether 327 duplex scans were analyzed and, for each, a resistive index (RI) was calculated. A raised RI predicted rejection in patients with grafts that functioned immediately, but not in those that had delayed function. A rise in intrarenal pressure ( greater than or equal to 40 mmHg) indicated the presence of rejection in both groups. However, neither test had a sensitivity of more than 71% and this was not improved by combining the results of the two tests for each patient. Although both tests have a place in transplantation, renal biopsies may still be required to confirm rejection.