Abstract AIM: To determine whether normal postnatal ultrasound, as part of a strict screening protocol for the detection and follow-up of antenatal hydronephrosis, effectively excludes the majority of babies with congenital urinary tract abnormalities that would otherwise present with a urinary tract infection. MATERIALS AND METHODS: We retrospectively reviewed all babies who had postnatal follow-up of antenatally detected hydronephrosis over a 5-year period at our institution, a district general Trust with a specialist paediatric unit. We then studied all babies presenting with urinary tract infection before their first birthday to our institution over the same period. By cross-referencing these two study groups we were able to determine which babies developed a urinary tract infection having been previously discharged after normal postnatal ultrasound. RESULTS: Four hundred and twenty-five babies had postnatal follow-up of antenatal hydronephrosis. Of these, 284 were investigated with ultrasound alone. In the same 5-year period, 230 babies presented with urinary tract infection before their first birthday. Only three of these babies had been previously discharged after normal postnatal ultrasound. The negative predictive value of a normal postnatal ultrasound was therefore 98.9% (281/284) for babies who subsequently presented with a urinary tract infection before their first birthday. CONCLUSION: Careful antenatal and postnatal ultrasound with strict protocols is effective in detecting congenital renal tract abnormalities. Infants discharged after normal postnatal ultrasound are highly unlikely to still have an undetected urinary tract abnormality. We suggest that all babies with antenatal hydronephrosis are started on prophylactic antibiotics at birth, pending further investigation. All babies without features of severe obstruction antenatally should have their postnatal ultrasound delayed for a month. We recommend selective use of micturating cystourethrogram (MCUG), and delaying this investigation until the baby is 3 to 4 months old to allow for spontaneous resolution of vesicoureteric reflux. We do not believe that all babies with antenatal hydronephrosis require MCUG, providing a clear protocol is followed.