Background: This study evaluated the clinical and economic impact of the emergence of third-generation cephalosporin - resistant Enterobacter species. Methods: Mortality, length of hospitalization, and hospital charges were examined in a cohort that was selected from a group of 477 patients with initial cultures that yielded a third-generation cephalosporin - susceptible Enterobacter species. Case patients (n=46) had subsequent cultures yielding a third-generation cephalosporin - resistant Enterobacter species. Control patients (n=113) who did not develop resistance were matched to cases on site of Enterobacter infection and length of hospitalization prior to isolation of the initial susceptible organism. Multivariable analyses were used to adjust for confounding. Results: Twenty-six percent of cases died vs 13% of controls (P=.06). The median total hospital stay for cases was 29.5 days (interquartile range [IQR], 20-60) and 19 days for controls (IQR, 13-27; P<.001). The median hospital charge for cases was $79 323 (IQR, $34 546-$161 384) and for controls was $40 406 (IQR, $18470-$79 005; P<.001). After adjusting for comorbidities, severity of illness, intensive care unit admission, surgery, transfer from another hospital, sex, and age, emergence of resistance was associated with increased mortality (relative risk, 5.02; P=.01), hospital stay (1.5-fold, P<.001), and hospital charges (1.5-fold, P<.001). Emergence of resistance had a median attributable hospital stay of 9 days and an average attributable hospital charge of $29 379. Conclusions: Emergence of antibiotic resistance in Enterobacter species results in increased mortality, hospital stay, and hospital charges. Minimizing resistance in Enterobacter species should be a priority.