The clinical efficacy, safety, and optimal dosage of oral ciprofloxacin for the treatment of respiratory infections were studied in a multicenter, open trial in Japan. Five hundred seventy-one patients with respiratory infections received orally administered ciprofloxacin (the majority received 600 mg daily). Clinical efficacy was evaluated in 542 of these patients, and safety was analyzed in 568 patients. Clinical efficacy was excellent or good in 80.9 percent of patients with pneumonia, and in 71.3 percent of patients with chronic airway infections. The overall bacteriologic eradication rate was 68.5 percent. In infections caused by Hemophilus influenzae, the rate was 90.8 percent. Side effects were observed in 4.6 percent of patients, and abnormal laboratory findings were noted in 6.9 percent; however, there were no severe reactions. Ciprofloxacin may be a very useful agent for the treatment of respiratory infections.