Abstract The clinical efficacy of ciprofloxacin was evaluated in 83 patients with chronic respiratory tract infection. Each patient received ciprofloxacin 200 mg TID for 2 or 4 weeks. After 2 weeks of treatment for acute exacerbation of chronic respiratory tract infection, 21 (52.5%) of 40 evaluable patients were considered “improved.” After 4 weeks of treatment, 24 (75%) of 32 evaluable patients were judged “improved.” An additional 2 weeks of treatment increased the rate of improvement. On the other hand, only 6 (23.1%) and 7 (26.9%) of 26 evaluable patients were considered “improved” after 2 and 4 weeks of treatment, respectively, for chronic inflammation. Ciprofloxacin was also administered to 6 patients for more than 60 days to examine its prophylactic effect against acute exacerbation. No patient had an acute exacerbation within 60 days of treatment. Ciprofloxacin is a useful oral antibacterial agent for the treatment and prophylaxis of acute exacerbation of chronic respiratory tract infection.