Background: Congenital kidney and urinary tract anomalies are the most common cause of chronic kidney disease in the first three decades of life. Stenosis of the ureteropelvic junction may cause dilation of the collecting system in the fetal kidney. This study aimed to determine hydronephrosis due to congenital ureteropelvic stenosis treatment outcome according to the age of the intervention. Methods: We conducted a retrospective descriptive study that included pediatric patients with hydronephrosis secondary to ureteropelvic junction stenosis operated by the Anderson-Hynes open pyeloplasty method from 2010 to 2016. Patients were divided into two groups: group A, children < 1 year of age, and group B, children > 1 year of age. We analyzed ultrasonographic parameters, renal function, and clinical data. Inferential statistics were used with the Mann-Whitney U-test and X 2 test. Intra-group data were assessed with the Wilcoxon test. Results: We included 52 patients: group A (n = 16, 30%) and group B (n = 36, 70%). The male sex predominated, and mainly the left renal unit. The most important surgical finding was stenotic segment. The median right glomerular filtration rate was 24.1 mL/min (19.0-34.5) pre-surgical and 38.2 mL/min (35.9-41.09) post-surgical in group A (p = 0.028), and 28.4 mL/min (18.5-35.0) pre-surgical and 37 mL/ min (35.7-46.0) post-surgical in group B (p = 0.003). The median left glomerular filtration rate was 30 mL/min (21.4-39.0) pre-surgical and 40.0 mL/min (37.7-44.6) post-surgical in group A (p = 0.005) and 18.4 mL/min (14.2-29.2) pre-surgical and 37 mL/min (33.1-38.5) post-surgical in group B (p < 0.001). Conclusions: Correction of ureteropelvic stenosis before one year of age results in better renal function than a later correction.