The relationship between resistive index of arterial Doppler waveform of the native kidney and serum creatinine level has been studied using duplex Doppler sonography in 40 patients with medical renal disease and 28 normal controls. The resistive index of the normal control group and the medical renal disease group was 0.627 +/- 0.042 and 0.823 +/- 0.237, respectively, with statistically significant difference. In the group with medical renal disease, there was a statistically significant correlation between the resistive index and the level of serum creatinine. The study suggests that duplex Doppler sonography may be helpful in evaluation of renal functional status in patients with medical renal disease.