Introduction and objective: To assess the usefulness of the Society of Fetal Urology (SFU) grading system, the urinary tract dilatation (UTD) classification, anteroposterior renal pelvis diameter (APRPD) measurement, and differential renal function (DRF) in 99mTc-EC scintigraphy (SC) for predicting long-term outcomes in children with ureteropelvic junction obstruction (UPJO). Materials and methods: Abdominal ultrasonography and SC at the time of UPJO diagnosis and at follow-up examination (initial/final US and SC) were evaluated. Initial and final blood pressure, serum creatinine (Cr), cystatin C, urine albumin-to-Cr ratio (ACR), and estimated glomerular filtration rate (GFR) were determined. Results: Fifty-three children with UPJO were studied. The median age at diagnosis was 0.81 years (0.10–6.01), and at follow-up examination, it was 5.17 years (1.75–11.60). Surgical treatment was required for 21 (40%) children, of whom 24% had an initial APRPD <20 mm, and 52% had an initial DRF ≥40%. Severe renal scars in the final SC were demonstrated in 17 (32%) children, of whom 47% had an initial APRPD <20 mm, and 41% had an initial DRF ≥40%. Hypertension was present in 3 (6%) patients, and laboratory symptoms of renal injury were observed in 6 (11%) patients. Receiver operating characteristic (ROC) analysis demonstrated low usefulness of the initial SFU and UTD classifications and DRF for predicting surgical treatment (area under the curve, AUC: 0.696, 0.728, 0,674, respectively) and severe renal scars (AUC: 0.772, 0.723, 0.662, respectively), An APRPD ≥19 mm demonstrated only moderate usefulness (AUC 0.822) for predicting surgery but was not useful for predicting severe renal scars. Conclusions: The ultrasonographic grading systems and DRF in renal scintigraphy at the time of UPJO diagnosis may not be sufficient for assessing adverse long-term outcomes in children.