Purpose : The increasing use of ultrasonography has allowed for an increase in the of the detection of congenital hydronephrosis, and the clinical outcomes of congenital hydronephrosis are widely varied. In this study, the necessity of voiding cystourethrography in neonate with hydronephrosis to rule out vesicoureteral reflux (VUR) was evaluated. Methods : Between January 2004 and December 2007, we reviewed the medical record of 157 childrens with congenital hydronephrosis detected within 1 month of age. The severity of hydronephrosis was graded by SFU (Society of Fetal Urology) system, and anterior posterior pelvic diameter (APPD). We evaluated the relationship between severity of hydronephrosis and incidence of VUR by using SPSS windows version 16.0. A P-value<0.05 is considered to be statistically significant. Results : Total renal unit number was 254, and 20(7.8%) renal units had VUR. We did not find any relationship between hydronephrosis grade the presence of VUR grade (P>0.05). In addition, there was no statistical significance between APPD, laterality of hydronephrosis and VUR incidence. However, renal units with VUR had lower spontaneous resolution rate (P<0.05), compared to renal units without VUR. Conclusion : In this study, there was no statistical significance between the severity of hydronephrosis and presence of VUR. Therefore, voiding cystourethrogram is recommended for all children with hydronephrosis to rule out VUR, regardless of the severity of hydronephrosis.