Most cases of reflux are a primary abnormality due to incompetence of the ureterovesical junction. The ureterovesical junction (UVJ) normally acts as a passive valve mechanism because of the oblique course of the intramural portion of the ureter. There is also a more active component with peristalsis of the longitudinal muscles of the ureter. Reflux results from immaturity or developmental abnormality of the ureterovesical junction. The length of the intramural portion of the ureter may be too short or insert at an abnormal position into the bladder. The normal insertion of the ureter is one quarter of the distance from the bladder base to the bladder dome.