Objective · To investigate the prospective value of early postoperative PI-APD in children with hydronephrosis.  Methods · Data of children with hydronephrosis who underwent pyeloplasty in Xinhua Hospital, Shanghai Jiao Tong University School of Medicine between Jan 2012 to Nov 2015 was collected. PI-APD was divided into 3 categories ( ≤ 19%, 19%<PI-APD<40% and ≥ 40%). The relationship between PI-APD value and the degree of renal function (DRF) and dilation recovery after surgery was analyzed.  Results · There were 360 children with hydronephrosis. The median follow-up was 20 months. The PI-APD value (3 months after pyeloplasty) was positively correlated with the degree of DRF recovery (r=0.631, P=0.000). Five patients received redo-pyeloplasty. PI-APD of all these patients was <19%.  Conclusion · PI-APD is a new feasible ultrasound parameter in pyeloplasty followup. PI-APD ≥ 40% at the first post-operative visit predicts pyeloplasty success. PI-APD ≤ 19% indicates close follow-up after operation. PI-APD can also help select children at high risk for repeat intervention after pyeloplasty.