BACKGROUND: Escherichia coli is the most common cause of gram-negative bloodstream infections, causing an estimated 40,000 deaths from sepsis each year in the United States. The present study sought to determine specifically which virulence factors (VFs) and phylogenetic groups of E. coli are epidemiologically associated with bacteremia. METHODS: E. coli isolates from 63 veterans with bacteremia and rectal isolates from 71 matched uninfected control subjects were compared both for phylogenetic group and for the presence of VFs and O antigens. RESULTS: Bacteremia isolates exhibited a significantly greater prevalence of most VFs studied. In multivariate logistic regression analysis, ompT (outer membrane protein T) was the strongest VF predictor of bacteremia (P<.001). Despite the concentration of most individual VFs within group B2, bacteremia and rectal isolates differed little by phylogenetic distribution, a finding explained by the greater prevalence of VFs among bacteremia isolates than rectal isolates within groups B2 and D. CONCLUSIONS: Although phylogenetic group partially corresponds with virulence potential in E. coli bacteremia, VFs are more-powerful predictors of pathogenic potential. Bacteremia isolates exhibit an arsenal of VFs that distinguishes them from rectal isolates from uninfected hosts, which makes these differences attractive potential targets in vaccine or drug development.