The Whitaker test, a urodynamic study, combined with antegrade pyelography has been used recently to evaluate persistent upper urinary tract dilatation after operative correction of obstruction. This test will differentiate patients with residual or recurrent obstruction from those with dilatation secondary to permanent changes in the musculature. It is useful in evaluating patients with questionable ureteropelvic or ureterovesical junction obstruction, or primary defects in the ureteral musculature, such as prune-belly syndrome. It also will establish when urinary diversion safely can be discontinued in postoperative patients. Percutaneous puncture of the renal pelvis is performed or an indwelling nephrostomy tube is used. The upper urinary tract is then perfused at a constant rate of 5--10 ml/min with saline or diluted contrast media, and a serial pressure recording is made in the renal pelvis and bladder. The high flow rate used will be tolerated easily in a nonobstructed system without a progressive rise in renal pelvic pressure. In obstructed systems abnormally high pressure above 12 cm water or a constant rise in pressure will be recorded. Videotaping of ureteral peristalsis and spot films of the upper urinary tract complete the evaluation.