We studied the role of duplex Doppler ultrasonography in the diagnosis of renal obstruction caused by ureteral calculi. Using duplex Doppler sonography, we evaluated the intrarenal hemodynamics of 27 patients who presented to the emergency department with renal colic. We performed Doppler ultrasonography on patients in whom US did not reveal any pathology causing renal colic and calculated and compared mean RI values of normal and obstructed kidneys and Delta RI values of each group. Threshold levels for the diagnosis of urinary tract obstruction (mean RI >= 0.70 and Delta RI >= 0.08) were used to determine the sensitivity and specificity of Doppler sonography for the diagnosis of urinary tract obstruction. Patients were investigated for revealing calculi diagnosis either by stone excretion history, intravenous pyelography or non contrast enhanced urinary computed tomography. A total of 162 intrarenal arterial Doppler recordings were made on 54 kidneys. Of the 16 patients with urinary obstruction, 11 (68%) had sonographic evidence of pelvicalyceal dilatation. The mean RI of the 16 obstructed and 11 unobstructed kidneys was 0.69 +/- 0.04 and 0.61 +/- 0.06 (mean +/- standard deviation), respectively. The difference between the mean RI values for each group was statistically significant (P < 0.05). Mean RI values of the contralateral kidneys in the obstructed group and unobstructed group were 0.61 +/- 0.03 and 0.59 +/- 0.05, respectively. Also Delta RI value (0.07 +/- 0.02) of obstructed kidney group was statistically higher than the Delta RI value (0.01 +/- 0.03) of the unobstructed group (P < 0.05). The mean RI of the 16 obstructed kidneys (0.69 +/- 0.04) was significantly greater than that of the 16 unobstructed contralateral kidneys (0.61 +/- 0.03) (P < 0.05). This study supplements the existing evidence that, in acutely obstructed kidneys, renal Doppler recording can demonstrate altered renal perfusion before pelvicalyceal system dilatation and distinguish obstructed and unobstructed kidneys evaluated with suspicion of renal colic.