With the widespread use of maternal ultrasound, the incidence of prenatally detected hydronephrosis has increased significantly altering the practice of urology. The recent review of the trends in the prenatal sonography use and subsequent urological diagnoses in United States demonstrated significant increase in the overall ultrasound use in the last two decades. Moreover, the mean number of ultrasounds per pregnancy also increased significantly from 2.7 in 1998 to 4.2 in 2005.1 Depending on diagnostic criteria and gestation, the prevalence of prenatally detected hydronephrosis ranges from 0.6% to 5.4%. The condition is bilateral in 17%–54%, and additional abnormalities are occasionally associated. The outcome of prenatally detected hydronephrosis depends on the underlying etiology. Although prenatally detected hydronephrosis resolves by birth or during infancy in 41%–88% patients, urological abnormalities requiring intervention are identified in 4.1%–15.4% and rates of vesicoureteric reflux (VUR) and urinary tract infections (UTIs) are several-fold higher. Pelviureteric junction (PUJ) obstruction is the most common cause of hydronephrosis detected antenatally.2 , 3 The next most common cause of prenatally detected hydronephrosis is obstruction at the ureterovesical junction.2 Management of these patients after birth remains controversial. The decision to intervene surgically in these infants has become more complex because spontaneous resolution of antenatal and neonatal upper urinary tract dilatations is being increasingly recognized.2 , 4 6 The recognition and relief of significant obstruction is important to prevent irreversible damage to the kidneys.7 Differentiating urinary tract dilatations that are significantly obstructive and require surgery from those that represent mere anatomical variants with no implications for renal function is not a simple task especially in the newborns. It has been shown that the changes in the function of the involved kidney should be used as a measure of degree of obstruction and indication for surgical intervention.4 , 5 , 7 10