S and F1C fimbrial adhesins often expressed by uropathogenic Escherichia coli are genetically homologous. A multiply primed polymerase chain reaction (PCR) was developed for discriminating the S (sfa) and F1C (foc) fimbrial operons. A total of 270 uropathogenic E coli strains and 80 fecal isolates were examined. PCR specifically detected the sfa and foc alleles in 105 (93%) of 113 sfa/foc+ strains by DNA hybridization. Furthermore, 87% of sfa+ uropathogenic E. coli simultaneously possessed the genes encoding the class III P fimbrial adhesin (prsG(J96)), alpha-hemolysin and cytotoxic necrotizing factor 1. Statistical analysis showed the class II P fimbrial adhesin (papG(IA2)) and F1C fimbria to be associated with high relative virulence in pyelonephritis and cystitis, respectively. The multiply primed PCR developed should be useful for assessing the contribution of the S and F1C fimbriae in the pathogenesis of urinary tract infections.