Introduction: Hydronephrosis is one of the most commonly diagnosed urinary tract defects in children. It is characterised by various degrees of dilation of the renal pelvis and calyces with concomitant thinning of the renal parenchyma. The dilation is caused by a ureteropelvic junction defect obstructing the outflow of urine from the kidney. Extreme hydronephrosis can lead to a complete lack of function of the affected kidney. The treatment of hydronephrosis involves restoring normal urine outflow from the kidney and depends on the cause of the condition. The decision to perform surgery depends on the rate of progression of abnormalities observed in the renal parenchyma, among other aspects. Aim of the study: The paper presents the experiences of a single centre in the surgical treatment of hydronephrosis in children aged up to 18 years. Materials and methods: In 2016–2020, 77 children underwent surgery for hydronephrosis at the present authors’ department. In 40% of cases, hydronephrosis was diagnosed on antenatal screening, in 31% it was observed on abdominal ultrasound performed due to abdominal pain, in 17% the condition was detected incidentally when the causes of other diseases were being investigated, in 9% urinary tract infection led to the discovery of hydronephrosis and in 3% of cases abdominal trauma was the reason the patient was examined in the first place. In 58% of the subjects, the cause of hydronephrosis was intramural stenosis of the ureteropelvic junction, in 22% it was the presence of accessory vessels and in 20% various other causes were found. In all patients, Anderson–Hynes ureteropyeloplasty was performed. Results: Surgical outcomes were assessed 12 months after the procedure, and in 97.4% of cases they were considered good. A repeat operation was performed in only 2 cases due to a lack of improvement after the original hydronephrosis surgery. Conclusions: Surgical treatment of hydronephrosis caused by ureteropelvic obstruction is an effective and safe method with a low risk of early and late complications.