OBJECTIVES: The purpose of the study was to estimate the incremental cost-effectiveness ratios (ICER) among the first-line antibiotics for the inpatient treatment of Mexican patients with community acquired pneumonia (CAP) from the healthcare payer's perspective. METHODS: A cost – effectiveness analysis was performed with the aid of a decision tree model. The model had a two-month period to assess economic and clinical consequences of seven first-line antibiotics used in the study: Gemifloxacin 320mg/day, Clarithromycin 1,000mg/day, Levofloxacin 500mg/day, Gatifloxacin 400mg/day, Ceftriaxone 1gr/day, Cefuroxime 1,500mg/day and Moxifloxacin 400mg/day. Adverse events of each therapy were considered (rash, diarrhea, dizziness, vomits, chill and head pain). Resource use data was obtained from hospital records from the General Regional Hospital No. 1 “Gabriel Mancera” at the Social Security Mexican Institute (IMSS) in Mexico City (n=94). All drug prices and unit costs were taken from official institutional databases within the IMSS. Effectiveness measures used in the model was the % of therapeutic success among the multiple antibiotics. One-way and probabilistic sensitivity analyses were performed and acceptability curves were constructed. RESULTS: Gemifloxacin showed the lowest average healthcare costs in inpatient treatment (US$ 2,389.8) due to a significant reduction in the hospitalization days (approximately 4.61 days in average per patient). The length of stay reduction was associated with the shorter treatment of gemifloxacin (five-days). Gemifloxacin showed the highest effectiveness (95.3%) followed by clarithromycin (94.3%); levofloxacin (94.0%); gatifloxacin (92.0%); ceftriazone (91.3%); cefuroxime (90.0%) and moxifloxacin (86.5%). The ICERxs for all treatments were dominated by gemifloxacin. First-order Monte Carlo simulations showed the same results. CONCLUSION: Gemifloxacin was the most cost effective first line treatment for hospitalized patients with CAP, especially, because it's high effectiveness and its significant inpatient-stay length reduction.