The increasing use and availability of renal transplantation has resulted in a demand for noninvasive methods to study possible complications. One of the most serious adverse reactions is acute rejection, a possibly reversible cause of transplant failure if treated promptly. Differentiation from other causes of acute renal failure frequently is difficult, and the lack of specificity in many imaging studies has been troublesome. Eighty-one patients with renal transplants, including 41 with acute rejection, were examined. Duplex Doppler examination of the intrarenal arteries and a simplified formula, the resistive index ([peak systolic frequency shift--lowest diastolic frequency shift]/[peak systolic frequency shift]), were used to diagnose rejection. With a resistive index greater than 0.90, a 100% positive predictive value was obtained for the diagnosis of acute rejection. A value less than 0.70 was unlikely to be rejection (negative predictive value, 94%). This approach uses a simple analysis of the waveform. Use of a duplex Doppler examination and the formula described here appears to be an accurate method for the detection of acute rejection and for the differentiation of acute rejection from the various other causes of acute renal failure.