The pelvi-ureteric stenosis (PU stenosis) is not uncommon in pediatric urology. Recently the conservative plastic operation is justified, but the choice of operative procedure is difficult at times. The purpose of this study is to observe histological changes of the kidney in PU stenosis to compare the morphologic findings with clinical data and prognosis. Renal biopsy specimens obtained from 32 children with hydronephrosis were examined. The light microscopic findings of the specimens were classified and scored according to the change in the renal interstitism, renal tubules and bloodvessels. The total score of each subject was designated as "Index of renal histologic alterations". The results were as follows: 1) Tne maturation of the glomerulus or nephron was not delayed or disturbed. 2) No renal dysplasia was found in the specimens. 3) In the group of giant hydronephrosis, moderate or severe change of the kidney structure was found in about half of the children, while in non-giant group, normal or slight alterations accounted for 80%. 4) Urinary tract infection was presentin over 70% of patients. Insidions infection was not uncommon. 5) When the biopsy specimen shows high Index of histologic alterations, one must be carefull of the persistent infection or urographic deteriorations. Our histologic examination of the kidney with PU stenosis in children revealed the relatively well preserved parenchyma. This evidence encouraged us to proceed the conservative plastic operation.