We investigated the clinical usefulness of the revised 2010 Clinical and Laboratory Standards Institute (CLSI) breakpoints for Escherichia coli and Klebsiella spp. Of 2,623 patients with bacteremia caused by E. coli or Klebsiella spp., 573 who had been treated appropriately with cephalosporin based on the CLSI 2009 guidelines were enrolled. There were no differences in the rates of treatment failure or mortality between the appropriately and inappropriately treated groups according to the CLSI 2010 guidelines. Additionally, in the matched case-control analysis, the treatment failure rate was higher in bacteremic patients with extended-spectrum β-lactamase- (ESBL-) producing but cephalosporin-susceptible organisms than in those with ESBL-nonproducing isolates when patients with urinary tract infections were excluded (44% and 0%, resp., P = 0.026). In patients with bacteremia caused by E. coli or Klebsiella spp., the revised CLSI 2010 guidelines did not lead to poorer outcomes. However, ESBL production appeared to be associated with poor clinical outcomes in patients with bacteremia from sources other than the urinary tract.