The prevalence of uropathogenic Escherichia coli bearing type 1 and/or p fimbriae was assessed in 179 adult women with urinary tract infections, and the presence of specific fimbriae types was correlated with results of localization studies. E. coli with p fimbriae occurred more frequently in patients with clinically defined pyelonephritis (13 of 23 [57%]) than in women with cystitis (22 of 116 [19%]; P = .0004) or asymptomatic bacteriuria (6 of 40 [15%]; P = .0008), whereas organisms with type 1 fimbriae were equally distributed in these three patient groups. In contrast, the presence of p-fimbriated strains was not correlated with infection localized to the upper urinary tract by either the antibody-coated bacteria technique (among symptomatic women) or ureteral catheterization (among asymptomatic women). Thus although p fimbriation seems to be an important virulence factor associated with development of acute pyelonephritis in adult women, its detection appears not to be a useful localization test per se, and efforts to prevent these infections should not be directed against this factor alone.