The method of diuretic scintigraphy was used to examine 27 children, aged 2-15 years, with excretion urography findings of dilatation of the upper urinary tract. Vesicorenal reflux was excluded in all the children of the series by classical radiologic cystography. The radionuclide 131I-hippurate was used in 15 children and 99mTc-MAG3 in 12 children. In both groups the parameter T 1/2 was determined as the time of 50% decrease of activity above the kidney after administration of the diuretic. With both radionuclides the same borderline time of T 1/2 was established. In cases with T 1/2 < 10 minutes and preserved kidney function, dilatation of obstructive origin can be excluded with both substances. 99mTc-MAG3, however, has a markedly better imaging capacity and a significantly lower radiation load than 131I-hippurate. (Tab. 4, Fig. 3, Ref. 12.)