In a prospective study of 50 consecutive patients with urographic renal pelvic dilatation, 30 with genuine ureteropelvic junction obstruction identified by 123iodine-hippurate diuretic renal scanning (renography) were treated by dismembered pyeloplasty. One patient with reduced preoperative function required secondary nephrectomy. Three patients with normal function in solitary kidneys had unchanged postoperative function. In the remaining 26 patients preoperative function measured by diuretic renal scanning was compared to 6-month postoperative values. If split function changes of less than 5 per cent were considered insignificant, to allow for inherent statistical errors of renal scanning, preoperative function was decreased in 18 patients, improved postoperatively in 10 and remained the same in 8. In 8 patients preoperative function was normal and remained so postoperatively. Drainage improved in 22 of the 26 patients and was unchanged in 4. Analysis of functional change by Student's t test and the Wilcoxon paired sums test showed significant improvement in mean and median function (p less than 0.001), with the degree of improvement being greatest in patients with the most severely depressed preoperative levels. In addition to producing significant improvement in drainage in ureteropelvic junction obstruction, pyeloplasty arrests functional deterioration in almost every case and improves function significantly in the majority.