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Objectives: The contribution of crossing vessels to ureteropelvic junction (UPJ) obstruction is controversial. We performed a pilot study on patients undergoing laparoscopic pyeloplasty in whom an intraoperative Whitaker test was performed before and after repositioning a crossing vessel to determine its effect on collecting system drainage. Methods: From August 2004 to July 2005, 10 patients with UPJ obstruction scheduled to undergo laparoscopic pyeloplasty were prospectively enrolled in this study. Routine laparoscopic access to the renal pelvis was obtained, and a crossing vessel, if present, was identified. Before mobilization of the UPJ or the crossing vessel, an intraoperative Whitaker test was performed using a laparoscopic 22-gauge needle to puncture the renal pelvis and infuse saline at a rate of 10 mL/min. Bladder and renal pelvic pressures were measured simultaneously. After complete mobilization of the UPJ and crossing vessel, if present, the Whitaker test was repeated. Dismembered pyeloplasty was then performed. Results: Of the 10 patients, 6 had crossing vessels and 4 did not. The Whitaker test was successfully performed in all patients. Among those with a crossing vessel, all had a crossing artery, and 67% also had a crossing vein. No significant change was found in the renal pelvic pressure after mobilization of the renal pelvis in patients without crossing vessels. In those with crossing vessels, the mean renal pelvic pressure significantly declined after vessel repositioning (25.6 ± 4.5 cm H2O to 9.5 ± 6.6 cm H2O, P = 0.006). Conclusions: In this pilot study, lower pole crossing vessels directly contributed to UPJ obstruction by causing extrinsic compression. [Copyright &y& Elsevier]