Postnatal imaging findings were reviewed in 130 neonates and young infants referred for imaging evaluation of antenatally detected renal abnormalities. All children underwent voiding cystourethrography and upper urinary tract imaging with sonography and/or renal scintigraphy. Vesicoureteral reflux was present in 49 patients (38%) and was bilateral in 24. All grades of reflux were observed. Reflux occurred in 41 of 98 neonates (42%) in whom postnatal imaging revealed persistent upper tract abnormalities (eg, hydronephrosis, cysts, renal agenesis) and in eight of 32 (25%) with normal findings at postnatal sonography and/or renal scintigraphy. Reflux was the single most common urologic diagnosis and was the only postnatal abnormality in 12 patients (9%). The authors conclude that neonates with antenatally detected hydronephrosis should be routinely screened for reflux with voiding cystography. Detection and aggressive management of reflux in the asymptomatic neonate in whom renal growth and function are unimpaired theoretically offer the best opportunity for preventing renal injury later in childhood.