What is vesico-ureteric reflux (VUR)? Urine flows from the renal pelvis through the ureter into the bladder. When the bladder is full, the detrusor muscle contracts and expels urine from the bladder through the urethra. In normal people, when the bladder contracts to urinate, urine does not flow back from the bladder into the ureter or kidney. However, 60-70% of infants with urinary tract infections have vesicoureteral reflux (Baker, 1966), in which a portion of the urine returns from the bladder into the ureter or even into the renal pelvis during urination. Why does vesicoureteral reflux cause nephritis? It is because a significant proportion of the bacteria of urinary tract infections (70-90% of E. coli) enter the bladder from the urethra. If the number is small, it is excreted in the urine with little or no symptoms. In large numbers, it can cause cystitis. The rate of urinary tract infection is 12 times lower in circumcised boys than in uncircumcised boys because the glans of the penis is exposed and relatively dry, so the amount of bacteria between the foreskin and the glans is greatly reduced and fewer bacteria enter the bladder through the urethra. The classic study by Ransley and Risdon in 1979 found that the combination of reflux and bacterial infection could damage the kidneys. When I went to London Children’s Hospital in 1995 to study pediatric urology, Ransley, who was working there at the time, took a year’s leave of absence and was not able to attend. Urinary infections in children are very common and are the second most common source of infection after respiratory infections. Five percent of fevers in children between two months and two years of age are caused by urinary tract infections. In the age group of one to five years, girls have a greater chance of urinary tract infections, 10-20 times more than boys. However, for infants under three months of age, boys are 5-8 times more likely than girls, which is related to the high incidence of congenital urinary tract malformations in males. Why is there vesicoureteral reflux? In normal children, the ureter enters the bladder at an angle, meaning that it travels diagonally through the bladder wall for a distance (5 times the diameter of the urethra, Paquin 1959) before entering the bladder cavity. When the bladder is full, the pressure of the bladder urine flattens the ureter within the bladder wall, closing the lumen and creating an anti-reflux mechanism. In children with reflux, the ureter enters the bladder at an angle close to right angles and loses its anti-reflux mechanism. When the bladder fills and contracts to urinate, the pressure in the lumen not only expels the urine but also causes vesicoureteral reflux.