Dilatation of the fetal urinary tract can now be detected by ultrasound imaging in pregnancy. Forty-six cases of pre-natally diagnosed dilatation were analysed to assess the contribution of this information to subsequent urological management. Positive pre-natal scans proved accurate. Dilatation was confirmed by neonatal ultrasound imaging or by autopsy in 24 of 25 consecutive inborn neonates and two aborted fetuses. Forty-four live births were studied. Twelve neonates had physical signs at birth which rendered the pre-natal diagnosis unnecessary. In 32 neonates (73%) the urinary tract anomaly was identified solely as a result of pre-natal imaging and would otherwise have remained undetected. However, in seven neonates the information was of doubtful value (mild unilateral dilatation), in 16 it was of probable value (confirmed unilateral PUJ obstruction, multicystic kidney, etc.) and in only nine neonates (20%) was it of definite value (treatable conditions affecting both kidneys or a solitary kidney, e.g. urethral valves, PUJ obstruction in a solitary kidney, etc.). Pre-natally detected bilateral dilatation (18 infants) carried a poor prognosis--33% mortality and 56% incidence of coexistent abnormalities. No infants in our series had undergone intra-uterine drainage, but the numbers of survivors and their renal function was comparable to the published results for fetal intervention.