This prospective study analyzed the intestinal carriage of P fimbriated Escherichia coli as a host susceptibility factor in urinary tract infection (UTI). P fimbriation was defined by the pap and G adhesin (papG1A2, prsGJ96) genotypes. Children with UTI carried pap+ E. coli in the fecal flora more often than healthy controls both at diagnosis (86% vs. 29%) and during infection-free intervals (approximately 40%; P < .01). P1 blood group-positive children carried pap+ E. coli in the fecal flora more often (88%) than those with P2 blood group (40%; P < .05). A pap+ E. coli strain caused UTI in 53 of 55 patients who carried both pap+ and pap- strains in their fecal flora. These results suggest that persons who develop UTI have an increased tendency to carry pap+ E. coli in the large intestine and that these pap+ E. coli cause UTI more often than pap E. coli strains in the fecal flora of the same host.