In vivo expression, in human urine, of P, type-1, and type-1C fimbriae by Escherichia coli was analyzed by indirect immunofluorescence. Urine samples from 20 patients with acute urinary tract infection, as well as cultures of the corresponding E. coli isolates, were immunostained with three polyclonal antisera to P fimbriae, antiserum to type-1 fimbriae, monoclonal antibody specific for type-1C fimbriae, and fluorochrome-conjugated second antibodies. P fimbriae were found in 17 urine samples and in 18 of the isolated strains. Type-1-fimbriate bacteria were detected in only nine urine samples, although 18 of the isolated strains expressed type-1 fimbriae after growth in vitro. Four strains possessed type-1C fimbriae; only two expressed type-1C fimbriae in urine. The bacterial populations in urine were heterogeneous, and in each positive staining, only a fraction of the bacterial cells were reactive. The results show that E. coli P fimbriae are expressed and are subject to phase variation in vivo during acute urinary tract infection.