To determine the clinical usefulness of Doppler ultrasonography in the diagnosis of the Nutcracker phenomenon, as an alternative to computed tomographic scans (CT). This study consisted of 52 patients that presented with intermittent hematuria of unknown origin between January 2006 to April 2008. Doppler ultrasonography was used to assess the left renal vein (LRV) by measuring the anteroposterior (AP) diameter and peak systolic velocity (PSV) in supine and standing positions, at the hilar and interaortomesenteric portions of the LRV. These data were compared with CT scans. The sensitivity and specificity of duplex sonography was determined using the AP diameter and PSV ratios to assess the cut-off levels. Kappa (k) statistic was also evaluated. mean AP diameters of the LRV measured by Doppler sonography were 8.38 mm at the hilar and 3.17 mm under the SMA, compared to 9.3 mm (hilar) and 3.2 mm (SMA) in the supine and standing position respectively. The PSV in the supine position was 25.77 cm/s and 115.48 cm/s, respectively, compared to 25.54 cm/s and 125.96 cm/s in the standing position. The cut-off levels were 3.85 (sensitivity: 61.5%, specificity: 80.8%, k:0.42) for the supine and 4.12 (sensitivity:61.5%, specificity: 65.4%, k: 0.27) for the standing AP diameter, 2.99 (sensitivity: 92.3%, specificity: 73.1%, k: 0.65) for the supine and 3.73 (sensitivity: 96.4%, specificity: 79.2%, k: 0.76) for the standing PSV. Our data show that the standing PSV ratio is the best parameter for to detecting entrapment of the LRV.