We present the findings of a prospective cohort study of babies born with antenatally detected urinary tract abnormalities (AUTAs) between 1999-2003 and compare the outcomes with those of an earlier cohort born between 1989 and 1993. All infants with a fetal anteroposterior renal pelvic diameter (APRPD) > or =7 mm in the third trimester or other urinary tract abnormality underwent a detailed postnatal ultrasound scan and other investigations as indicated. The incidence of AUTAs was significantly greater in the more recent cohort (7.6/1000 vs. 3/1000 live births; p<0.05). Of the 350 infants on which we had data, 48.6% (170/350) were in the non-specific dilatation (NSD) category, and vesicoureteric reflux (VUR) was detected in 12%. Restricting investigations to those who had an APRPD > or =10 mm at >30 weeks of gestation could have reduced the number with NSD in the more recent cohort (26/115; 25%), but 25% of those with pelviureteric junction hold-up and 50% with VUR would have been missed. Significantly fewer patients in the more recent cohort underwent surgery (7 vs. 21%; p<0.001). There is a trend towards larger APRPDs on third trimester scans being associated with more significant pathology, but there is a lot of clinical overlap. The study highlights the need for cautious antenatal counselling combined with an assurance to prospective parents that postnatal investigations will be performed in a stepwise manner based on the initial postnatal ultrasound scan and clinical findings.