Objective In the Netherlands standard ultrasound screening at 20 weeks of pregnancy was introduced in 2007. Since then the number of prenatally detected abnormalities is increasing. Urinary tract abnormalities are found. Hydronephrosis is the most frequent;y detected condition. The significance of this dilatation for the child after birth is still not fully clear. Methods 34 Dutch centers for prenatal diagnostics were asked to fill in a questionnaire about their policies concerning prenatally detected hydronephrosis. We also asked for input from pediatric nephrologists and urologists from the 8 Dutch university hospitals and some of the larger general hospitals, about their postnatal follow-up strategies of these babies. Results 22 centers for prenatal diagnostics returned the questionnaire (65%). 77% consider an AP diameter of the renal pelvis <5 mm at 20 weeks of pregnancy normal. At 32 weeks of pregnancy in 73% of the centers a diameter <10 mm is considered normal and in 27% of the centres abnormal. Postnatal follow-up varies a lot in the different hospitals. Ultrasound of the kidneys and bladder, voiding cystography and renography are considered the standard diagnostic tools for follow-up. There is a wide variation in timing of and indication for the different investigations. Conclusions There still is a debate about the significance of prenatally detected hydronephrosis. Prospective studies are needed to determine accurate follow-up protocols and cut-off values. This is important for the selection of those babies who need medical care and to prevent unnecessary interventions for the babies not at risk for renal pathology.