Three-quarters of human kidneys have segments that will allow intrarenal reflux if the person is one of the 1% that is also born with vesicoureteric reflux (VUR). It is likely that entry of infected urine into these segments produces permanent damage within just a few days, as it does in piglets and adult pigs. This very rapid course leaves no time for delay in diagnosing and treating urine infections in infants, the group that present the greatest clinical difficulties. It is proposed that the reason why the risk of scarring starts off high and falls to virtually nil by 4 years is not due to maturation that leads to an increased resistance to scarring, but because most vulnerable subjects have already scarred their kidneys in infancy. This proposed model has important implications for clinical management. First, it suggests that current practice identifies scars in children due to urine infection, but prevents few. Second, babies known to have VUR from birth, and protected from scarring with prophylactic antibiotics, will not outgrow their scarring risk by any particular age, but will remain at risk until they outgrow their reflux. This suggests their kidneys need to be protected from scarring until then, perhaps by antibiotic prophylaxis. Third, transplant recipients of any age with refluxing ureteric anastamoses or stents will carry a risk of developing a focal scar if they acquire a urine infection, and may need protection.