Objective:To assess the clinical outcome and imaging features of neonatal pri mary vesicoureteral reflux(VUR).Study design:We prospectively followed 43 inf ants with primary VUR identified from among a cohort of 497 infants with fetal r enal pelvis dilatation.Postnatal renal ultrasound(US)examinations were perfor med at 5 days and 1,3,6,12,and 24 months of life.Voiding cystourethrography was performed in the neonatal period and repeated at 12 and 24 months when VUR was persistent.Two radioisotopic examinations,including a 99mTc-MAG3 renogram and a plasma clearance of Cr-51 EDTA,were performed in all children with high-grade reflux.Results:The incidence of primary VUR in our study group was 9%.Among the 43 patients followed,11(26%)had high-grade(IV-V)VUR and 32(74%)had low-grade VUR.Resolution of reflux occurred in 2 of 11(18%)patien ts with high-grade VUR and in 29 of 32(90.6%)patients with low-grade VUR at age 2 years(P < 0.001).At age 2 years,91%of the low-grade refluxing kidne ys were normal on US,compared with only 35%of the high-grade refluxing kidney s.Split renal function was within normal range and single-kidney GFR was signi ficantly increased in 13 of the 17 high-grade refluxing kidneys during follow-up.Furthermore,a strong association between dysplasia on US and poor renal fun ction outcome was found.Conclusions:In most infants with VUR,the reflux is of low grade and resolves rapidly.In those children with high-grade VUR,spontan eous resolution is rare at age 2 years,but persistent reflux rarely impairs the maturation of renal function. Objective: To assess the clinical outcome and imaging features of neonatal pri mary vesicoureteral reflux (VUR). Study design: We prospectively followed 43 infants with primary VUR identified from among a cohort of 497 infants with fetal r enal pelvis dilatation. Postnatal renal ultrasound (US) examinations were performed med at 5 days and 1,3,6,12, and 24 months of life. Moving cystourethrography was performed in the neonatal period and repeated at 12 and 24 months when VUR was persistent. Two radioisotopic examinations, including a 99mTc-MAG3 renogram and a plasma clearance of Cr-51 EDTA, were performed in all children with high-grade reflux. Results: The incidence of primary VUR in our study group was 9%. Among the 43 patients followed, 11 (26 (18%) patien ts with high-grade VUR and in 29 of 32 (90.6%) had high-grade (IV-V) %) patients with low-grade VUR at age 2 years (P <0.001) .At age 2 years, 91% of the low-grade refluxing kidne ys we re normal on US, compared with only 35% of the high-grade refluxing kidney s. Split renal function was within normal range and single-kidney GFR was signi ficantly increased in 13 of the 17 high-grade refluxing kidneys during follow-up. Furthermore, a strong association between dysplasia on US and poor renal funtion outcome was found. Conclusions: In most infants with VUR, the reflux is of low grade and resolves rapidly.In those children with high-grade VUR, spontan eous resolution is rare at age 2 years, but persistent reflux rarely impairs the maturation of renal function.