We used immunoblotting to examine the serologic antibody responses to outer membrane proteins (OMP) of Escherichia coli in both symptomatic and asymptomatic elderly subjects with urinary tract infections. Controls with no present or past urinary tract infections showed variable weak immunoglobulin G (IgG) antibodies to OMP of infecting strains. Elderly individuals with asymptomatic infections demonstrated antibody to both lipopolysaccharide (LPS) and OMP of their infecting strain, with consistent cross-reactivity to OMP of other infecting strains. Young females with acute pyelonephritis showed an IgG response to LPS and OMP with cross-reactivity to OMP of other strains. Elderly individuals with symptomatic invasive infections had strong reactions to both LPS and OMP in specimens collected during the acute phase, generally with an increase in intensity in specimens from convalescent patients. They also demonstrated extensive cross-reactivity to LPS and OMP from all other infecting strains. IgM antibody was not observed in any patients. These data confirm other reports of low levels of antibodies to OMP of E. coli in normal populations. Asymptomatic bacteriuria in this population is associated with antibody responses to the LPS and OMP of the infecting strain. Elderly individuals with invasive infections had initial reactions to the infecting strain with an apparent increase in intensity during convalescence. Antibodies to the major OMP appear to be broadly cross-reactive.