To determine whether recurrent, symptomatic urinary tract infections (UTIs) in a given individual are due to the same or different strains, 71 Escherichia coli strains that caused recurrent UTIs were prospectively collected from 23 infection-prone young women and studied by chromosomal restriction fragment length polymorphism (RFLP) analysis using pulsed-field gel electrophoresis. Thirty-five strains from women with first-episode UTIs were also studied. Overall, 30 (68%) of 44 recurrent UTIs were caused by a strain previously identified in that person. In contrast, 32 of 35 strains from first-episode UTIs had unique RFLP profiles. Analysis of a subset of subjects established that the majority of recurrent UTIs were due to reinfection, not persistence of the pathogen within the urinary tract, and suggested that the colonic flora was the reservoir for these reinfecting strains.