We reviewed 39 neonates with prenatally diagnosed duplex system anomalies. Principal diagnoses were ureterocele in 15 patients, ureteral ectopia in 15, lower polar vesicoureteral reflux in 6, lower pole ureteropelvic junction stasis in 2 and yo-yo reflux in an incompletely duplicated system in 1. Several patients had other ipsilateral and contralateral urinary anomalies. Ten patients (26%) had relevant physical signs and only 1 became symptomatic, with urosepsis, neonatally. At initial assessment, before any accounted urinary tract infection, renal polar function as judged by radionuclide examinations almost always was severely impaired in the presence of major ureteral ectopia or severe reflux. By contrast, function was usually well preserved when there was obstruction, or lesser degrees of ectopia or reflux. A total of 19 patients underwent surgery (14 electively and 5 after episodes of urosepsis), while 20 have been managed nonoperatively for 12 to 84 months (mean 34) during which time the appearances and function of the upper renal tracts have remained stable. We conclude that the natural history of these anomalies is often benign and that a policy of routine surgical intervention, developed when most cases presented symptomatically, may not be appropriate in patients who present antenatally.