Yo-yo renal pelvis: an unusual cause of flank pain.

Herein we describe 2 patients in whom a bifid renal pelvis was associated with peristaltic dysfunction and pyelopelvic reflux, causing pyelocaliectasis and flank pain. The patients were improved with pyelopyelostomy. Previous explanations for the yo-yo phenomenon have indicated that there is functional obstruction because of a higher resting pressure in the common stem. The asynchronous peristaltic waves a coincident with a relatively low pressure in the oal fold that acted as a baffle to direct the flow of urine from the lower segment of the bifid pelvis down the ureter. This baffle favored reflux into the upper segment.