Background There is still a lack of knowledge regarding the natural course of resolution of hydronephrosis after pyeloplasty, and no consensus exists on how resolution of hydronephrosis is defined or when resolution is expected to occur. Objective To determine when resolution of hydronephrosis occurs following pyeloplasty, by type of obstruction and by surgical approach. Methods This retrospective study included 125 children age <15 years treated with pyeloplasty and followed for two years with repeated ultrasound and MAG3 scan. Children with single kidneys, bilateral disease, and without hydronephrosis were excluded. Children with reinterventions were excluded in the evaluation of hydronephrosis but not in terms of success rate. Outcomes time to resolution of hydronephrosis (Anterior-Posterior diameter (APD) <10 mm or >50% reduction of APD) and 2-year success rate. Exposure was surgical approach and type of obstruction (intrinsic/extrinsic). Survival analysis was performed, adjusting for age, gender, year, laterality, preoperative renal function on MAG3, calyces dilatation and APD in the multivariable analysis. Results At 12 months and 24 months follow-up, 90% and 93% had reached resolution, respectively. All children with persistent dilatation had improved drainage and stable