A comparative study of the resistivity indices (RI) obtained in both kidneys has been carried out in 30 controls and 60 patients with suspected unilateral acute obstruction of the upper urinary tract. Doppler findings (difference in the mean RI of both kidneys, delta RI) in the pathologic population have been systematically correlated to the data yielded by intravenous pyelography (IVP). In the control population delta RI was always < or = 0.03 (mean delta RI = 0.01, SD = 0.01). Considering that an increase in delta RI > or = 0.05 (> mean RI + 3 SD) is significant, 93% sensitivity and 100% specificity are obtained for the diagnosis of unilateral acute obstruction. Comparison of the resistivity indices of both kidneys is more specific and more sensitive than the assessment of the index on the obstructed side only; it improves the performance of ultrasonography in the initial diagnosis of acute urinary tract obstruction.