Ultrasonography (US) of the native kidneys is commonly requested for acute renal failure (ARF), although in most cases the examination results are negative. To detect changes in the Doppler waveform associated with ARF and determine whether Doppler US can provide significant diagnostic information not available with standard US, 91 patients with ARF were studied to determine a mean resistive index (RI) for each patient. Forty-six patients had acute tubular necrosis (ATN) with a mean RI +/- 1 standard deviation of .85 +/- .06, which was significantly higher than the mean RI of .67 +/- .09 in 30 patients with prerenal ARF (P less than .01). Fifteen patients had ARF due to non-ATN intrinsic renal disease (mean RI, .74 +/- .13). An elevated RI (greater than or equal to .75) occurred in 91% of patients with ATN versus only 20% of patients with prerenal azotemia. Patients with severe liver disease (hepatorenal syndrome) are a subset of those with prerenal ARF that accounted for most of the elevated RIs in this group. The study demonstrates that intrarenal Doppler US allows detection of changes associated with ARF far more often than standard US. More important, Doppler US may be helpful in distinguishing ATN from prerenal azotemia.