Norfloxacin (NFLX), an oral antibacterial agent of new quinolone derivative, was administered at daily doses of 300-600 mg t.i.d. to 20 aged patients with respiratory tract infections. The results obtained are summarized as follows. 1. Clinical efficacies were moderate in 6 patients with upper respiratory tract infections, and moderate in 11 patients and fair in 3 patients with lower respiratory tract infections. In the 20 patients, overall clinical efficacies were moderate in 17 patients, hence the efficacy rate was 85%. 2. Bacteriologically, causative organisms were detected in 10 of the 14 cases with lower respiratory tract infections. In these cases, bacteriological responses were "eradicated" in 9 cases, "decreased" in 1 case and "unknown" in 1 case, the rate of eradication was 90%. 3. No adverse reactions or abnormal laboratory test values were observed.