Mild fetal pyelectasia, defined as a renal pelvic anteroposterior (AP) diameter of 4-10 mm, has become a frequent finding on fetal ultrasonography. The natural history of such dilatation is unclear, resulting in confusion as to appropriate postnatal investigation and management. The aim of this study was to examine the urinary tract outcome of a series of infants with mild fetal pyelectasia demonstrated on routine morphology ultrasonogram between 16 and 21 weeks' gestation. Of the 37 cases identified, 13 (35%) went on to require medical or surgical intervention for significant urinary tract anomalies. These anomalies included pelvi-ureteric junction obstruction, dysplastic kidney, vesicoureteric reflux and posterior urethral valves. On initial scan all cases had an AP diameter of 4-8 mm and did not predict those infants who would go on to require intervention. An AP diameter of greater than 7 mm on repeat scans performed after 27 weeks' gestation had a positive predictive value of 0.92 and a negative predictive value of 0.76 for significant urinary tract anomaly requiring intervention. The specificity was 0. 94 and sensitivity 0.70. A protocol of one repeat antenatal ultrasound at 28-34 weeks' gestation would be able to identify those infants who would require postnatal investigation, using a measurement of >/=7 mm. The fetus with a normal repeat ultrasound would not require postnatal follow-up.