Although hydronephrosis detected by prenatal ultrasonography often is assumed to be secondary to obstructive uropathy, in approximately 10% of the cases renal pelvic dilatation results from primary vesicoureteral reflux. More than 80% of neonates with reflux are male and two-thirds have bilateral reflux. Approximately 80% have at least grade III reflux. At birth between a third and half may have reduced renal function on isotope renography, even in the absence of urinary infection. Approximately 20% of neonates with grade IV or V reflux followed nonoperatively experience spontaneous reflux resolution by age 2 years. However, in approximately 25% of boys followed nonoperatively urinary tract infections developed by age 2 years despite antimicrobial prophylaxis. Because the majority of these boys have been uncircumcised, circumcision seems advisable. Ureteral reimplantation should be reserved for those with breakthrough urinary tract infection, new renal scars or persistent high grade reflux.