Hydronephrosis; Ureter; Congenital; Laparoscopic surgery; Hydronephrosis severity score; Ureteropelvic junction stricture Background: Hydronephrosis is the most common urogenital anomalia in children. Renal UPJ obstruction is the most common cause for upper urinary tract obstruction occurring 1 in 2000 births. Urinary tract obstruction occurs due to blockage of urine flow along the urinary tract and generally manifests as hydronephrosis on imaging. Kidney function impairment from urinary tract obstruction, if present, is readily reversible if the obstruction is promptly corrected. Uncorrected UTO can damage the kidney tissue and lead to progressive kidney function impairment and end-stage kidney disease. Urinary tract obstruction in children may be acute or chronic, partial or complete, and unilateral or bilateral. The cause of urinary tract obstruction may be congenital or trauma. Hydronephrosis in children mostly congenital and localization of obstruction may in upper part and lower part of ureters, 90% of obstruction’s cause were upper part of ureters. In Mongolia we use ultrasound andurogramm /Radiologic test/ to identify the localization and severity of obstruction. Hydronephrosis can only be treated surgically, either laparoscopically or through open correction of ureteropelvic junction. We don’t have a research paper about causes, clinical significant and results of surgical treatment for congenital hydronephrosis. Aim: The aim was to determine clinical sign, diagnose and compare surgical outcome of laparoscopic and open pyeloplasty. Objectives: 1. Clinical signs of hydronephrosis in Mongolian children 2. Study the results of a new method of hydronephrosis severity assessment scores in children 3. To compare the outcome of surgical treatment laparoscopically or through open correction of ureteropelvic junction. Material and methods: In the period from 2020 to 2022. We have operated 30 ureteropyeloplasty cases. All the patients had UPJ obstruction and ureteropyeloplasty was performed. Both groups were compared according to the operative time and recovery duration. Demographic data including age, gender, operation time, estimated blood loss, hospital stay and complications were recorded. Results: Mean age was 6.1±4.0 in all the study children. A total of 30 patient, n=25 (83.3%) boy, n=5 (16.7%) girl were enrolled in this study. In this study, a new hydronephrosis severity score (HSS), combining ultrasonographic and renographic parameters, has been developed. Hydronephrosis severity score was analysed with regard to its usefulness in assessing the severity of UPJO. There was 60.3% agreement between traditional and new hydronephrosis severity score (p=0.014). Compared to that open surgery, blood loss and hospital stay 2 days shorter than open surgery. But duration of operation time was longer than open surgery (162.3±65.5). There was statistical different (p=0.002) between parameters of 2 groups. In laparoscopic and open surgery group respectively, renal function was increased both groups and there was statistical different between 2 groups. Conclusions: The most common cause of with upper ureteric blockage hydronephrosis in children is congenital ureteropelvic junction stricture was occurred 94.7% in all cases and abdominal pain is main clinical symptom in Mongolian children. There was 60.3% agreement between traditional and new hydronephrosis severity score. Compare laparoscopic surgery to open surgery had less blood, fewer hospital stays and faster wound healing. But operation time was longer than open surgery. Renal function was more improved laparoscopic surgery group after 6 months.