Assuming a local weakness in the defence system to be responsible for chronic, recurrent infection of the urinary tract in young girls, we made 86 determinations of the bacterial periurethral flora from 70 female patients aged between five and eleven years. Within three patient groups (urologically healthy, temporarily free from infection, currently suffering an acute attack of chronic, recurrent urinary tract infection), we found gram-negative bacteria in 1/20 of the healthy children, in 1/2 of those temporarily free from infection and in all children suffering an acute attack. Enterococci (the accompanying flora) was found in 1/20, 1/2 of the children respectively. In both cases, the differences were significant (chi 2-Test, p less than 0.01). In comparison to healthy children, the more children, in 1/2 of those temporarily free from infection and in all children suffering an acute attack. Enterococci (the accompanying flora) was found in 1/20, 1/2 of the children respectively. In both cases, the differences were significant (chi 2-Test, p less than 0.01). In comparison to healthy children, the more children, in 1/2 of those temporarily free from infection and in all children suffering an acute attack. Enterococci (the accompanying flora) was found in 1/20, 1/2 of the children respectively. In both cases, the differences were significant (chi 2-Test, p less than 0.01). In comparison to healthy children, the more frequent colonization of children who were not currently infected, but who were prone to infection, was seen particularly as an indication of an intermittent local weakness in the defence system, possibly occurring when a defence mechanism occasionally falls short of a threshold level, thus allowing an infection to set in.