Between 1986 and 1995, 24 newborn and young infants underwent to surgical repair of 32 renal units affected by congenital hydronephrosis in spite of their levels of renal function and dilation. 22 renal units were pelviureteral junction obstruction and 10 were vesico-ureteral obstruction. Isotopic studies were made pre- and post-operatively, resulting in a high percentage of kidneys with the highest values of relative renal function showing a downward after surgery, and a general improvement among the renal units with previously moderate function. Drainage slope curves showed an improvement of its partial obstruction, and no change when dilation without obstruction. The clearance half-time showed a reduction in the slower renal units. We conclude that the natural evolution of these age-group kidneys is a counterbalance with the healthy contralateral renal unit.