Hydronephrosis is the most common abnormal finding in the urinary tract on prenatal screening with ultrasonography (U/S). Hydronephrosis may be obstructive or non-obstructive; obstructive lesions are more harmful to the developing kidneys. The aim of the study was to evaluate the causes of renal pelvic dilatation and the outcome of postnatal treatment in infants with hydronephrosis diagnosed prenatally with U/S. We prospectively studied 67 (60 males) newborns with hydronephrosis diagnosed prenatally and confirmed postnatally with U/S from Sept. 2005 to Oct. 2007. The patients were allocated to three groups based on the mea-surement of the anteroposterior renal pelvic diameter (APRPD) in transverse plane: mild (6-9.9 mm), moderate (10-14.9 mm) and severe (> or =15 mm) hydronephrosis. Voiding cystourethrography (VCUG) was obtained in all of the patients to rule out vesicoureteral reflux (VUR). In cases with negative VUR, Diethylenetriamine-pentaacetic acid (DTPA) scan with diuretic renography was performed to detect ureteropelvic joint obstruction (UPJO). Twenty two cases (32.8%) had mild, 20 (29.9%) had moderate, and 25 (37.3%) had severe hydronephrosis. The causes of hydroneph-rosis were VUR (40.2%), UPJO (32.8%), posterior urethral valves (PUVs) (13.4 %), and transient hydronephrosis (13.4 %). The lesion was obstructive in 37 (55.2%) infants. Totally, 33 (49.2%) patients with hydronephrosis (9 mild, 9 moderate, and 15 severe) subsequently developed com-plications such as UTI and renal insufficiency, or required surgery. Associated abnormalities were observed in 15 (22.4%) patients. We conclude that every newborn with any degree of hydro-nephrosis should be assessed postnatally for specific diagnosis and treatment.