Standardized evaluation of all newborns with antenatally recognized hydronephrosis (ANH) at The Hospital for Sick Children (HSC) has included voiding cystourethrography (VCUG). This paper reviews this protocol to determine: (1) the prevalence of vesicoureteral reflux (VUR) in isolated ANH and (2) the value of performing VCUG in cases of mild hydronephrosis, defined as renal pelvis dilatation <10 mm on postnatal ultrasonography (US). A retrospective chart review was performed on infants referred with ANH. The inclusion criterion was isolated ANH. Exclusion criteria were (1) presence of additional genitourinary abnormalities and (2) no VCUG. Pelviectasis was categorized according to the anteroposterior diameter of the renal pelvis. There were 111 infants with isolated ANH. All except 3 underwent VCUG. There were 68 children (63%) with normal postnatal US or mild pelviectasis (<10 mm). VUR was detected in 16 patients, of whom 10 had mild or absent pelvic dilatation. There was no correlation between the degree of pelviectasis on postnatal US and the presence or severity of VUR ( P=0.567 and P=0.802). VUR was detected in 15% of children with isolated ANH, many of whom had normal postnatal US or mild postnatal pelviectasis. VCUG is the only reliable test for detecting postnatal VUR.