40 adult patients with cystic fibrosis (CF) were admitted to hospital with acute exacerbations of infection associated with isolation of Pseudomonas aeruginosa from sputum. The patients were randomly allocated (20 per group) to receive intravenous azlocillin 5 g and gentamicin 80 mg, or oral ciprofloxacin 500 mg. Both treatments were given three times a day for 10 days. The patients were assessed on days 1 and 10, and at 6 weeks. There was a significant improvement in lung function between days 1 and 10 in both groups (p less than 0.001). Significant improvement was maintained at 6 weeks after ciprofloxacin but not in the intravenous group. Improvement after ciprofloxacin was superior at day 10. Sputum weight decreased in both groups (p less than 0.001). Patient-recorded symptoms also improved in both groups. There was no serious toxicity or side-effects. Drug resistant organisms were isolated no more frequently after ciprofloxacin than after intravenous therapy. 17 of the ciprofloxacin-treated patients said they preferred oral treatment to intravenous therapy. Oral ciprofloxacin is a useful short-term treatment for patients with CF who are infected with Ps aeruginosa.