The purpose of the study was to evaluate pyeloplasty a.m. Anderson-Hynes in the treatment of hydronephrosis in a large series of patients. In 175 patients, consecutively operated for ureteropelvic junction obstruction, we found 24 (14%) with minor postoperative complications, mostly urinary tract infection. Six patients (3%) were reoperated because of continued pain and obstructed drainage postoperatively. There was a small, but statistically significant increase in renal functional share on the operated side. We found a highly significant correlation between postoperative changes in total GFR and single kidney GFR on the operated side, which suggests a real improvement in renal function, rather than a simple reallocation of function between the two kidneys. We conclude that newer treatment modalities known as "minimally invasive surgery" must give comparably good results in order to be an acceptable alternative to traditional dismembered pyeloplasty.