Normally, urine flows down from the kidneys and passes through tubes called the ureters, which then enter the bladder. When a child has vesicoureteral reflux, the urine flows in the wrong direction into the kidney, which causes the bacteria that normally leave the body in the urine to come back into the kidney. This, in turn, causes pylonephritis, which is an infection of the kidney.

Approximately 30 to 40 percent of children who have a urinary tract infection are found to have reflux. In most cases, it is a febrile UTI, meaning the child has a high fever, resulting from a kidney infection.