This retrospective observational study evaluated the impact of antimicrobial consumption on antimicrobial susceptibility among aerobic Gram-negative bacteria after introducing ertapenem to the formulary of a teaching hospital (1130 beds) in northern Taiwan. Data on consumption of various antimicrobial agents, expressed as defined daily dose/1000 patient-days (DDD/1000 PD), were collected retrospectively from hospital pharmacy records 2 years before and 5 years after the introduction of ertapenem (October 2005). During the study period, the consumption of ampicillin and aminoglycosides decreased significantly. In contrast, the consumption of cefoxitin, ceftazidime, cefpirome, piperacillin-tazobactam, carbapenems (ertapenem, imipenem, and meropenem), and fluoroquinolones (ciprofloxacin, levofloxacin, and moxifloxacin) increased significantly over time. There was a significant increase in the rate of susceptibility of Escherichia coli to ampicillin, cefotaxime, ceftazidime, piperacillin-tazobactam, cefpirome, amikacin, and levofloxacin; an increase in the rate of susceptibility of Klebsiella pneumoniae to ceftazidime, cefepime, cefpirome, piperacillin-tazobactam, meropenem, levofloxacin, and amikacin; a significant decrease in the rate of susceptibility of Pseudomonas aeruginosa to meropenem; and a significant decrease in the rate of susceptibility of Acinetobacter baumannii to ceftazidime, carbapenems, ciprofloxacin, and levofloxacin. The rate of antibiotic susceptibility to ertapenem of extended spectrum β-lactamase producers, including E. coli and K. pneumoniae, remained stable. Usage of ertapenem was found to be negatively and significantly associated with the susceptibility rates of P. aeruginosa to meropenem and gentamicin. Significantly negative correlations were noted between the use of ertapenem and the rates of susceptibility of A. baumannii to ceftazidime, piperacillin-tazobactam, carbapenems (imipenem and meropenem), ciprofloxacin, and levofloxacin.