The diagnosis of urinary tract obstruction is a difficult and perplexing problem particularly in children. Pyelocalyectasis is seen not only in obstruction but also in other conditions, such as residual dilatation afler relief of obstruction, vesicoureteral reflux and pyelonephritis. Grey-scale ultrasonography is of little value in this clinically impOliant distinction. The standard excretory urography (IVU), even with diuretic augmentation, does not permit the objective diagnosis or exclusion of urinary tract obstruction. The Whitaker test is considered by some authors to be the gold standard for the diagnosis of obstructive pyelocalyectasis but it is invasive and therefore has not gained wide use. Moreover, the intrinsic urine output of the kidney contributes an unknown volume to the total amount of fluid being infused, particularly in children, and the potential for false-positive results should be considered whenever the urine output of the corresponding kidney is high. Finally, the results are not always reproducible or consistent with surgical findings. Nowadays, diuretic renography is the most widely accepted non-invasive procedure to diagnose obstruction. However, it has the disadvantages of being expensive, using ionising radiation and having a 10% -15% rate of false-positive and indeterminate results (Kass et aI., 1985). Magnetic resonance imaging (MRI) (Thumher et aI., 1989) and various biochemical indicators of the response of the kidney to obstructive damage (Carr et aI., 1994) have recently been investigated. However, the clinical significance of such new approaches remains to be determined.