Objective To investigate the relationship between the expression of aquaporin-2(AQP2) in hydronephrotic kidney(HNK)and urine in children with congenital hydtonephrosis.Methods Twenty patients with congenital unilateral pyeloureteric junction obstruction(PUJO)were chosen for this study,in whom 10 suffered mild hydronephrosis and 10 suffered severe hydronephrosis,with the mean age of 61.2±30.7 months.Eight nortnal ehildren served as controls,with the mean age of 58.9±34.2 months.The expression of AQP2 in HNK tissue was determined by immunohistochemisty,and the AQP2 levelin urine was determined by enzyme-linked immunosorbent assay(ELISA).Results Positive immunoreactivity of AQP2 was found in the apical membrane and intracellular vesicles of the principal cells in collecting ducts.The decreasing of HNK tissue and urine AQP2 expression level was consistent with the decreasing of urinary osmotic pressure in patients with hydronephrosis.The AQP2 expression in severe hydronephrotic group was significantly lower than that in mild hydronephrotic group(P＜0.05),and AQP2 expression in abovementioned two groups was significantly 10wer than that in the controls(P＜0.05).A positive correlation between the expression level of AQP2 in HNK tissue and urinary AQP2 level,and urinary osmotic pressure were found(r=0.706,P＜0.05;r=0.810,P＜0.05).Conclusions With the malignancy of hydronephrosis,the expression of AQP2 in HNK tissue and the urinary AQP2 levels and the urinary osmotic pressure have parallel decreasing,indicating the change of urinary AQP2 level may be used as a marker to predict the change of the expression of AQP2 in congenital HNK tissue,thus it may provide evidence for evaluating the urinary concentrating capacity.