The presence or absence of ureteral peristalsis was noted during real-time sonography of 61 dilated ureters in children. The findings were correlated with diagnoses established using standard radiographic and radionuclide imaging techniques. Of the 47 dilated ureters that exhibited peristalsis 44 were classified as not obstructed when assessed with standard imaging and functional studies. The most frequent etiology for ureteral dilatation associated with peristalsis was high grade vesicoureteral reflux (31 ureters). Three peristaltic ureters were shown to be mildly to moderately obstructed. Absence of peristalsis was noted in 14 ureters: 13 were severely obstructed, while in 1 the involved kidney had no function. In the pediatric age group the demonstration of peristalsis in a dilated ureter is frequently associated with vesicoureteral reflux and is seldom associated with obstruction. Obstruction, if present, usually is mild. Aperistaltic ureterectasis implies severe obstruction or poor renal function.