The efficacy and safety of gatifloxacin (GFLX) was evaluated for elderly patients with respiratory infections. Each patient received one-half (100 mg b.i.d.) or one-quarter (100 mg q.d.) of the conventional dosage of 200 mg b.i.d., after a tentative clinical dosage for GFLX was estimated based on the patient's age and body weight. The subjects were 34 patients aged 65 years or older with mild to moderate acute bronchitis, pneumonia, or chronic respiratory tract infections. The serum concentration of GFLX was measured for each patient, and population and pharmacokinetic (PPK) analysis was performed, using the Bayesian method, to calculate the AUC and maximum drug concentration (Cmax). The overall efficacy rate of GFLX for 33 patients was 87.9% (29/33 patients). GFLX was effective for 75.0% (6/8 patients) in the 100-mg dosage group and 92.0% (23/25 patients) in the 200-mg dosage group. The clinical efficacy was 90.0% (9/10 patients) for acute bronchitis, 86.7% (13/15 patients) for pneumonia, and 87.5% (7/8 patients) for chronic respiratory tract infections. The bacterial eradication rate was 85.7% (12/14 patients). No adverse events or laboratory abnormalities were observed. The AUC values were 11.2-37.5 microg.h/ml and 12.7-111 microg.h/ml for the 100-mg and 200-mg dosage groups, respectively, and the Cmax values were 1.28-3.02 microg/ml and 0.72-6.35 microg/ml, respectively, for the two groups. These results suggest that the dosage of GFLX examined in this study is clinically useful in elderly patients aged 65 or older with acute bronchitis, pneumonia, or chronic respiratory tract infections. The results of PPK analysis with the dosage management also support the efficacy of GFLX.