Prenatally or postnatally diagnosed dilatation of the upper urinary tract initiates postnatal investigations, including sonography, dynamic renography (MAG 3) and optional voiding cystourethrography. The first ultrasound examination should be performed 3-5 days after birth and in urgent cases 10-12 h after delivery of the baby. Dynamic renography (MAG 3) is the standard diagnostic investigation and permits simultaneous assessment of renal perfusion and drainage. MRI combines excellent anatomical detail with functional information, without exposure to radiation but needs general anaesthesia in infants and younger children. Intravenous urography is no longer performed routinely in children with a dilated upper urinary tract. The combination of ultrasound and MAG 3 produces the necessary anatomical and functional information to follow the degree of obstruction and to decide between surgical intervention or a conservative follow-up.