OBJECTIVES: Community-acquired intra-abdominal infections are serious conditions and expensive events. The purpose of this study was to develop an economic model in order to evaluate the cost-effectiveness ratios between ertapenem and the patterns of antibiotics used routinely in the treatment of community-acquired intra-abdominal infections at the Social Security Mexican Institute (IMSS) from the health care payer's perspective. METHODS: A cost-effectiveness analysis was developed using a Bayesian decision-tree model. The model simulates costs and effectiveness outcomes in a 4-week period. The comparators were: ertapenem(1 g daily); metronidazole (500 mg every 8 h)/amikacin (1 g every 12 h); metronidazole (500 mg every 8 h)/cefotaxime (1g every 8 h); metronidazole (500 mg every 8 h)/ciprofloxacin (500 mg every 12 h); metronidazole (500 mg every 8 h)/ceftriaxone (1 g every 12 h). Resource use and cost data were obtained from clinical records (n=53) of patients being treated at second-level hospitals at IMSS. Effectiveness measures were the percentage of clinical success without adverse events (AE) at the end of the follow-up period. Effectiveness data and transition probabilities were taken from international published literature and were adjusted according to the antimicrobial susceptibility identified locally. The model was calibrated according to international pharmacoeconomics guidelines. One-way and probabilistic sensitivity analyses were performed using Monte Carlo Simulation second-order approach. RESULTS: Patients who received ertapenem experienced 74.4% of clinical success without AE, followed by metronidazole/amikacin (52.6%) and metronidazole/cefotaxime (40.6%). Mean cost per patient was lower for ertapenem (US$6,293.98) followed by metronidazole/amikacin (US$6,830.78) and metronidazole/cefotaxime (US$8,511.03). Regarding the ICER's, ertapenem resulted the dominant therapy. Acceptability curves showed ertapenem as the most cost-effective therapy achieving values close to 100% independently of IMSS willingness to pay. CONCLUSIONS: The results showed that in México, ertapenem was the most cost-effective antibiotic therapy for community-acquired intra-abdominal infections. These results should be taken into account by Mexican decision makers.