Thirty-three patients (thirty-five kidneys) underwent pyeloplasty for hydronephrosis due to obstruction of ureteropelvic junction between 1963 and 1982 in our department. Renal function was judged to have improved in 68 and 67% of the patients from the radiogram and radioisotope renogram, respectively, Anderson-Hynes pyeloplasty was performed on 27 kidneys. In these kidneys, the improvement of renal function as judged from the radiogram and radioisotope renogram was 74 and 71%, respectively. Postoperative results were compared according to age, preoperative urinary tract infection (UTI), indwelling periods of nephrostomy and splint catheter, and duration of postoperative UTI. In addition the relationship between indwelling periods of nephrostomy catheter and the duration of postoperative UTI are discussed. Infants under 10 years old had the best improvement. Preoperative UTI and indwelling periods of nephrostomy and splint catheter were not directly related with post operative results. The cases with nephrostomy catheter which could be removed within the 16th day after operation had excellent improvement on UTI and also in renal function.