Recently, proper use of antimicrobial agents for treatment of infections based on pharmacokinetics (PK)/ pharmacodynamics (PD) theory has been recommended to many clinical doctors. To consider the optimized administration method based on PK/PD theory for the elderly patients of gatifloxacin (GFLX), which was one of the oral respiratory quinolone antibacterial, Monte Carlo Simulation was conducted with community-acquired pneumonia and susceptibility data for Streptococcus pneumoniae (345 strains) collected by the second gatifloxacin surveillance study. From this results, the probabilities of achieving AUC/MIC 30 against pneumococcus for the elderly patients was 96.3% at twice-daily dosing of GFLX 100 mg, 97.2% at twice-daily dosing of GFLX 200 mg, therefore, it was suggested that the clinical effectiveness of the regimen of GFLX 100 mg was as well as the regimen of GFLX 200 mg.