Objective To examine aquaporin 1-4(AQP1-4)expression in the hydronephrotic kidney(HnK)with congenital pyeloureteral junction obstruction(PJO)in children and correlation to the degrees of hydronephrosis.Methods The diseased kidney specimens of 22 cases(male 15,female7)of PJO were collected during the operation of pyeloplasty and nephrostomy between July.2006.andJune 2007.The age ranged from 1 mon to 12yr(mean 58.95±54.27 mon).According to the visualization of intravenous pyelography(IVP),the HnKs were divided into the mild group(n=10;visualization within 60 minutes)and severe group(n=12,visualization over 60 minutes).Eight renal samples from the normal part of kidney of Wilm's tumor selected as controls.1mmunohistochemistry was applied to determine the expressions and locations of AQP1-4.The mRNA levels of AQP1-4 were determined by semi-quantitative reverse transcription polymerase chain reaction(RT-PCR).Results AQP1positive immunostaining was found in the endothelium of capillary of glomerulus,and the endothelium of proximal tubule and the descending limb of Henley's loop.AQP2 positive immunostaining was demonstrated in the endothelial layer of collecting tubule.The positive immunostaining of AQP3 andAQP4 were found in the epithelial cells of basal layer and the basal endothelial layer of Henley's loop respectively.The expression of AQP1-4 was significantly decreased in HnK group as compared with thenormal kidneys.RT-PCR revealed that the AQP1-4 levels of severe HnK group were lower than those of mild HnK group and the normal controls(AQP1:0.194±0.118 vs.0.598±0.092 vs.0.858±0.122;AQP2:0.247±0.089 vs.0.566±0.105 vs.0.976±0.134;AQP3:0.426±0.126 vs.0.741±0.074 VS.1.006±0.084;AQP4:0.171±0.115 VS.0.420±0.081 VS.0.739±0.201,P＜0.01 respectively).Conclusions The decreased expressions of AQP1-4 are related to the degrees of hydronephrosis.This would be associated with damaging to the renal concentrated and diluted functions during the development of hydronephrosis.