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Introductions. The widespread use of prenatal ultrasonography results in increased recognition of congenital hydronephrosis, a therapeutic and diagnostic challenge. This study was conducted to investigate the natural course of prenatal hydronephrosis and the accuracy of postnatal APD in determining the outcome. Materials and Methods. All newborns with prenatal hydronephrosis were followed up by ultrasonography after birth. Voiding cystoureterography, diethylene triaamine pentaacetic acid renal scintigraphy, and dimercaptosuccinic acid renal scintigraphy were done if indicated. The receiver operating characteristic curve was plotted to determine the best cutoff for the anterior-posterior pelvic diameter (APD) to distinguish surgical from spontaneously resolving group. Results. Of 178 neonates, 42 (23%) required surgery. The area under the curve for APD to predict the need for surgery was 0.925 with an APD cutoff of 15 mm. The diagnostic value of APD for determining the need for surgery was determined by sensitivity and specificity of 95.2% and 73.5%, respectively. Conclusions. Postnatal APD on ultrasonography has a valuable diagnostic accuracy for requiring surgery and provides a useful guide for parental counseling.