Over a 15-year period, 19 patients with hydronephrosis due to pelviureteric obstruction associated with lower polar vessels were treated by ureterolysis and transposition of the vessels. Only those patients who, at operation, showed rapid pelvic emptying was not observed, a pyeloplasty was performed in addition. Complete relief of symptoms occurred in 90% of the patients. Post-operative reduction in the degree of hydronephrosis was demonstrated by intravenous urography in 74% and renal pelvic drainage was judged excellent by diuresis renography in 80%. Diuresis renography allowed the demonstration of free and rapid pelvic drainage in the majority of those patients with persisting hydronephrosis shown by intravenous urography.