There is great need for an oral agent that could be used to treat pulmonary exacerbations in patients with cystic fibrosis. In this study, the use of oral ciprofloxacin as sole therapy was evaluated in 18 patients with 39 infectious episodes; 13 episodes were classified as severe, 19 were classified as moderate, and seven were classified as mild. Patients ranged in age from eight to 36 years (mean, 23 years). Dosage varied according to severity of disease, body size, and the susceptibility of the Pseudomonas isolate to ciprofloxacin; the dose ranged from 750 to 2,250 mg daily (mean, 1,800 mg). Ten patients received one course of ciprofloxacin, and eight received repeated courses. The overall clinical response rate was 82 percent. There was a response to the initial treatment course in 96 percent of the patients. Those in whom therapy failed had been re-treated with ciprofloxacin and were severely ill. Failure to respond correlated poorly with pretreatment minimal inhibitory concentration (MIC) values (0.6 microgram/ml for failures versus 0.4 microgram/ml for responses). Pseudomonas could not be eradicated from the sputum of any of the patients, although there was a marked reduction in purulence and bacterial counts. In general, patients who did not require re-treatment for three months would again have susceptible organisms. When organisms became resistant to ciprofloxacin (MIC greater than 2 micrograms/ml), they showed no concomitant new aminoglycoside or beta-lactam resistance. No serious toxicity occurred in any of the 39 episodes of treatment. In seven patients treated with combination therapy (tobramycin or azlocillin), the infecting organisms were reduced in number, but eradication of Pseudomonas generally could not be achieved. Increases in MIC occurred during combination therapy. Ciprofloxacin is a major advance in the treatment of bronchopulmonary infection in patients with cystic fibrosis.