In clinical pediatric urology distinction between obstructive and non obstructive dilatation of the upper urinary tract represents a difficult task for which usual procedures, both radioisotopes and conventional radiologic methods, have not discriminatory value. The aim of this investigation in to suggest a method of study that contributes to establish, in children, true diagnosis in clinical situations of hydronephrosis and to study evolution of renal function in surgical patients. Suggested technique, known an isotopic diuretic urography (UID), consists in intravenous administration of a diuretic (Furosemide, 0.4 mg/kg body weight) thirty minutes after injection of a radiotracer (I-123 Hippuran and or Tc-99-DTPA) into the vascular space and recording and analysis of variations of activity in parenchyma and renal pelvis during, at least, the following 60 m. Method described has been used in a series of 24 patients suffering from uni or bilateral hydronephrosis, with ages ranging from 2 months to 12 years. Results obtained, have shown that UID is notably superior to actually existing non-invasive methods of study of obstructive uropathy.