The presence of type 1 fimbriae, P fimbriae and X adhesins in 30 strains of Escherichia coli producing pyelonephritis, 30 strains producing cystitis, and 30 strains producing recurrent urinary infections are compared. All the strains were collected from adult women without risk factors for infections of the urinary tract. The presence of P fimbriae was 73% in strains producing pyelonephritis, 40% in strains producing cystitis and 20% in strains producing recurrent urinary infections; the presence of X adhesins was 3%, 0, and 17% respectively. There were, in consequence, significant differences between the groups studied; however, such differences were not observed with respect to the presence of type 1 fimbriae, which was 97%, 97% and 90%, respectively. We conclude that P fimbriae act as a factor of greater virulence in the strains of E. coli producing pyelonephritis and X adhesins are found almost exclusively in the strains producing recurrent urinary infections.