Objective To explore the relationships of β₂-microglobulin (β₂-MG), retinol binding protein (RBP) and aquaporin-2 (AQP2) in serum and urine with the severity of hydronephrosis induced by ureteropelvic junction obstruction (UPJO) in children. Methods Sixty children with hydronephrosis who underwent surgery for UPJO at the Department of Urology of the Sixth People's Hospital of Nanning were selected as hydronephrosis group. According to the severity of hydronephrosis, they were divided into mild group (n=13), moderate group (n=22), and severe group (n=25). Meanwhile, 60 healthy children in the same period were selected as the control group. The serum and urine levels of β₂-MG, RBP and AQP2 were compared between control group and children with hydronephrosis of hydronephrosis group. The correlation between the levels of β₂-MG, RBP, AQP2 and ultrasound parameters[anteroposterior diameter of renal pelvis(APD), parenchymal thickness (PT)]was analyzed. The receiver operating characteristic (ROC) curves were used to analyze the diagnostic efficacy of β₂-MG, RBP and AQP2 in serum and urine for moderate to severe hydronephrosis. Results The levels of β₂-MG and RBP in serum and urine of the hydronephrosis group were higher than those in the control group, and the levels of AQP2 in serum and urine were lower than those in the control group (P < 0.05). The levels of β₂-MG inserum and urine, RBP in urine, and APD from low to high in different groups were as follows: mild group, moderate group to severe group. Urine AQP2 level and PT from high to low in different groups was as follows: mild group, moderate group to severe group (P < 0.05). Serum RBP level in the mild group was lower than those in the moderate group and the severe group (P < 0.05). Urinary AQP2level was negatively correlated with APD and positively correlated with PT (P < 0.05). ROC curve analysis results showed that serum β₂-MG, urine β₂-MG, urine RBP, urine AQP2 detection had certain diagnostic value in moderate and severe hydronephrosis, and the area under the curve (AUC) was 0.652, 0.688, 0.859, 0.680, respectively. The AUC of urinary RBP in the diagnosis of moderate and severe hydronephrosis was significantly higher than that of other indexes (P < 0.05). Conclusion β₂-MG in serum and urine, urine RBP and urine AQP2 are related to the severity of hydronephrosis induced by UPJO in children. Urine RBP level has better diagnostic efficacy for moderate to severe hydronephrosis than other indicators.