The back flow of URINE from the URINARY BLADDER into the ureters (see URETER). The cause is defects in the VALVES which normally prevent this reflux from occurring. If, in addition to reflux, the patient – usually a child – acquires bacteria in the bladder, the consequence may be one or more attacks of PYELONEPHRITIS caused by the infected urine gaining access to the kidney pelvis. Diagnosis is by imaging techniques. Treatment is by long-term antibiotics while awaiting spontaneous recovery. Occasionally, corrective surgery is required.