Unilateral hydronephrosis due to obstruction of the uretero-pelvic junction is one of the most common indications for scintigraphic evaluation of the kidneys in the pediatric patient. In these cases, dynamic renal scintigraphy-both standard and after diuretic administration--is a simple and well-established procedure, allowing demonstration of structural and functional abnormalities of the involved kidney, with relatively low radiation dose to the child. Scintigraphic procedures allow the differential diagnosis between obstructive and non obstructive causes as well as an estimation of renal function of the hydronephrotic kidney providing indication for a conservative or a surgical approach.