Purpose: To determine the various prenatal factors related to the prenatal hydronephrosis diagnosed on prenatal ultrasonography. We also attempted to correlate the prenatal and neonatal renal pelvic anteroposterior diameter with the outcome in infancy Methods: Between 1985 and 1997. We retrospectively reviewed 105 renal unit (75 patient) with fetal hydronphrosis persisting postnatally. Investigation consisted of renal ultrasonography, voiding cystourethrography, diurectic renogram, and DMSA scan. Results: The 75 patient assesed had the following underlying cause: UPJ(Ureteropelvic juction) obstruction(52%). multicystic dysplastic kidney(10%). UVJ (Ureterovesicai juction) obstruction (10%) and no underlying cause (25%). Of theses cases 36 cases (40 renal unit) underwent operation, while 28 cases (50 renal unit) resoled spontaneously. 12% of mild hydornephrosis deteriorated. whereas 50% of modrate hydrophrosis and 81% of severe hydronephrosis required surgical correction. Attempting to find the renal unit that were at risk for deterioration. our study showed that urinary tract infection group and calyceal blunting group had a predictive role. Conclusion: It is necessary to follow up after birth dilatation of caylx or urinary tract infection are present. Early operation is considered when prenatal pelvic AP diameter greater is than 22 mm and postnatal diameter greater than 17 mm. This may make it possible to prevent further progression of renal damage and prompt treatment of asymptomatic hydronephrosis before complications occur.