Antenatal hydronephrosis is one of the most common congenital urological anomalies identified on prenatal ultrasound. There are controversial results in the postnatal management of mild-moderate isolated antenatal hydronephrosis. The purpose of this study was to investigate the clinical outcome and the frequency of urinary tract infection in infants with mild-moderate isolated antenatal hydronephrosis during the first year of life. The patients with mild-moderate hydronephrosis on the first renal ultrasonography done between seven days and two weeks of age were included in this study. The patients with other kidney abnormalities other than hydronephrosis, ureteral dilatation or bladder abnormalities were not included in the study. Hydronephrosis was classified as mild (5-9.9 mm), moderate (10-14.9 mm) by anterior-posterior pelvic diameters. 140 patients [96 boys (68.6%), 44 girls (31.4%)] were included in the study. Sixty (42.9%) patients had mild hydronephrosis and 80 (57.1%) patients had moderate hydronephrosis. The rate of spontan resolution was higher in patients with mild hydronephrosis than other group [n=58 (96.6%), n=48 (60%), respectively, p<0.01]. The median regression time was shorter in patients with mild hydronephrosis [median regression time; 4 (3-5.25); 6 (5-7) months, respectively, p<0.01]. The frequency of urinary tract infection did not differ between the patients with mild and moderate hydronephrosis patients (p>0.05). There was no evidence of acute pyelonephritis and obstructive or progressive hydronephrosis. Mild and moderate isolated antenatal hydronephrosis is often a self-limited condition. Children with mild and moderate isolated antenatal hydronephrosis might be followed without antibiotic prophylaxis.