Purpose: The aim of this study was to evaluate the diagnostic accuracy of renal pelvic dilatation (RPD) for detecting infants with antenatal hydronephrosis (AHN) who will need surgical intervention for ureteropelvic junction obstruction (UPJO) during follow-up. Patients and Methods: Between 1999 and 2010, 371 neonates were diagnosed with isolated AHN and were prospectively followed. The main event of interest was the need of pyeloplasty. Diagnostic odds ratio (DOR), sensitivity, specificity, and diagnostic accuracy (assessed by the area under the receiver operating characteristic curve, AUC) of fetal RPD and postnatal RPD were evaluated. Results: A total of 312 patients were included in the analysis. Twenty-five (7.5%) infants underwent pyeloplasty. The diagnostic performance for detecting the need of pyeloplasty was excellent for all ultrasonography measurements. The AUC was 0.96 (95%CI, 0.92 - 0.98) for fetal RPD, 0.97 (95%CI, 0.95 � 0.98) for postnatal RPD, and 0.95% (95%CI, 0.92 - 0.97) for SFU grading system. A cut-off of 18 mm for fetal RPD and a cut-off of 16 mm of postnatal RPD had the best DOR to identify infants who needed a pyeloplasty. Considering a diagnosis to be positive only if fetal RPD was > 18 mm and postnatal > 16, the sensibility was of 100% and the specificity was of 86% (95%CI, 81.5 � 89.9). Conclusion: Our findings suggest that the combination of fetal and postnatal RPD is able to increase the diagnostic accuracy for detecting infants who need a more comprehensive postnatal investigation for upper urinary tract obstruction.