This study was performed to evaluate the role of postnatal ultrasonography (US) in predicting the final diagnosis and need of surgery of antenatal hydronephrosis patients. One hundred and twenty six renal units (RUs) of 76 patients with diagnosis of antenatal hydronephrosis (ANH) were studied. An early postnatal US no later than the first week of life was requested. Voiding cystourethrography (VCUG) and/or diuretic renogram (MAG3) was performed in children who had persistent or worsening hydronephrosis to make a certain diagnose of etiology. US findings of different etiologies, and operated/nonoperated groups are compared. Regarding the renal pelvic anteroposterior diameter in the first week postnatal US; mild, moderate, severe, and no HN was detected in 48 (38.1%), 31 (24.6%), 24 (19.0%), and 23 (18.3%) RUs respectively. Eight RUs with a normal first postnatal US were found to have renal anomaly during follow up and 63% of these were vesicoureteral reflux (VUR). Kidneys with mild or moderate HN were likely to have transient dilatation while severe HN was likely to have obstruction. Postpartum follow-up of AHN is a dynamic process. Follow-up must be planned with optimum period of intervals according to clinic and US findings, to select the right patient for surgery or close follow-up.