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 ad hesin (papG_1A2 ,prsG_J96) 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 (∼40%; P < .01). PI 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.