Urine normally flows in one direction--down from the kidneys, through tubes called ureters, to the bladder. Vesicoureteral reflux (VUR) is the abnormal flow of urine from the bladder back into the ureters. VUR is most commonly diagnosed in infancy and childhood after the patient has a urinary tract infection (UTI). About one-third of children with UTI are found to have VUR. VUR can lead to infection because urine that remains in the child's urinary tract provides a place for bacteria to grow. But sometimes the infection itself is the cause of VUR. There are two types of VUR. Primary VUR occurs when a child is born with an impaired valve where the ureter joins the bladder. This happens if the ureter did not grow long enough during the child's development in the womb. The valve does not close properly, so urine backs up (refluxes) from the bladder to the ureters, and eventually to the kidneys. This type of VUR can get better or disappear as the child gets older. The ureter gets longer as the child grows, and the function of the valve improves. Secondary VUR occurs when there is a blockage anywhere in the urinary system. The blockage may be caused by an infection in the bladder that leads to swelling of the ureter. This also causes a reflux of urine to the kidneys.