The tissue transit time (TTT) in the 99mTc- MAG3 diuretic renogram seems to be a predictive factor for the deterioration of the differential renal function in these patients. We therefore analyze the tissue transit time in the 99mTc- MAG3 diuretic renogram in a prospective multicenter observational study to determine it as a prognostic and diagnostic factor. Aim of the study is to differentiate which children could be treated conservatively and which will benefit from operative pyeloplasty