Background The cortex/ pelvis ratio or parenchymal to pelvicalyceal area ratio can help initial evaluation and follow up of the children with uretropelvic junction obstruction as simple modality using the cut off which is 1.6 (pelvis/ cortex) ratio. There are many studies evaluating the various preoperative parameters which influence the post-operative outcomes after pyeloplasty. Practically there are many ultrasound parameters in the post-operative period which indicate the change in renal function. So, our rationale in this study is to evaluate the role of the parenchyma hydronephrosis area ratio (PHAR) in initial evaluation and follow up of UPJO patients in correlation to renal isotope studies. Objective To evaluate sonographic assessment of Renal Parenchyma to Hydronephrosis Area Ratio (PHAR) in the initial management and follow up of UPJO patients in correlation with the renal scan. Patients and Methods Site of the study: All patients were selected from the children who attended our outpatient department and have proven UPJO and meeting the criteria for the need of surgical intervention at Faculty of Medicine, Ain Shams University. Sample Size: 36 patients. Using PASS II Program for sample size calculation and according to (Rickard et al,2016), the expected AUC for PHAR prediction of future surgical intervention = 0.18, 30% rate of surgical intervention, setting power at 8% and error at 0.05, sample size of 36 patients will be needed. Results We enrolled 36 cases of children ≤ 16 years old (7.93 ± 0.83) with unilateral UPJO. Preoperative ultrasound parameters of the hydronephrotic kidneys [including degree of hydronephrosis, parenchymal thickness and PHAR] and renal isotope are measured & compared with the same parameters after surgical repair of the UPJO (Anderson Hynes pyeloplasty) at 3rd months. After operation, 31 cases (86.1%) improved & 5 cases not improved (13.9%). We find a significant change in all parameters at 3 months postoperatively (p < 0.01) as T ½ (25.22 ± 2.49 & 17.57 ± 3.84) significantly decrease & parenchymal thickness (9.42 ± 4.92 & 15.12 ± 4.86), GFR of the affected kidney (34.31 ± 3.31 & 48.32 ± 6.99) split renal function (37.30 ± 3.80 & 44.03 ±4.11) and PHAR (0.86 ± 0.30 & 2.45 ± 0.93) significantly increase post operatively. PHAR post operatively shows positive correlation with parenchymal thickness & renal isotope while negative correlation with T ½. Conclusion PHAR is a promising parameter that can be estimated on presentation US to help predict the future need for surgery and follow up in children with ureteropelvic junction obstruction. Early identification of children who may require surgical intervention allows for closer surveillance of those patients as well as the opportunity for more selective ordering of invasive testing.