The resistive index (RI), calculated from the duplex Doppler waveform, was compared with clinical and laboratory findings and the results of renal biopsy in 41 patients with nonobstructive (medical) renal disease. Kidneys with active disease in the tubulointerstitial compartment had a mean RI of 0.75 +/- 0.07. This was statistically significantly different (p less than .01) from the RI in kidneys with disease limited to the glomeruli (mean RI of 0.58 +/- 0.05). Acute tubular necrosis resulted in an elevated RI (mean RI = 0.78 +/- 0.03) as did vasculitis/vasculopathy (mean RI = 0.82 +/- 0.05). Patients with hypertension, proteinuria, or hematuria did not have kidneys with a significantly higher RI than did patients without these clinical factors. Kidneys found to be abnormally echogenic did not have an RI significantly different from kidneys of normal echogenicity. There was a weak correlation between creatinine level and RI value, reflected by a linear correlation coefficient of 0.34. In patients with normal renal RIs, the mean creatinine level was 1.7 +/- 1.7, whereas in those with abnormal RI values (greater than or equal to 0.70), the mean creatinine level was 3.7 +/- 3.6. We conclude that some forms of nonobstructive renal disease can produce changes in the Doppler waveform detectable by RI measurement. The production of Doppler waveform changes is strongly influenced by the site of the main disease within the kidneys. Active disease within the tubulointerstitial compartment (acute tubular necrosis, interstitial nephritis) or vasculitis/vasculopathy generally resulted in an elevated RI, whereas disease limited to the glomeruli, no matter how severe, did not significantly elevate the RI. Degree of renal dysfunction as indicated by serum creatinine level probably affects the Doppler waveform to some degree, but the relationship is weak.