Antenatal hydronephrosis (AHN), defined as dilatation of renal pelvis and/or calyces, is the most frequently detected antenatal abnormality. However, postnatal management of AHN is controversial. The purpose of this study was to describe the clinical outcomes of infants with AHN and to contribute to the definition of the postnatal evaluation of these patients. One hundred and thirty-six infants with AHN were prospectively followed up to 18 months. Patients were divided into two groups according to the degree of sonographic hydronephrosis (HN) on days 5-7: group I (n = 87, 64%) included patients who had grades 1 and 2 (64%) and group II (n = 49, 36%) included patients who had grade 3 and above HN. The grade of HN was found to be correlated with the increased risk of urologic pathologies. Frequency of vesicoureteral reflux was found to be significantly lower in patients with mild HN (6%) as compared to patients with severe AHN (29%) (p = 0.005). In addition, the risk of urinary tract infection increases with increasing grades of HN (10% vs. 29%, p = 0.006). The frequency of spontaneous resolution in patients with mild AHN (64%) was also significantly higher than in patients with severe HN (29%) (p < 0.001). The degree of AHN can be used for making decision about further diagnostic imaging and treatment. Our results strongly suggest that low-grade HN is a relatively self-limited condition and needs minimal investigation. In contrast, the outcome of more severe degrees of AHN needs clarification.