The single-dose pharmacokinetics of oral ciprofloxacin were studied in ten patients with cystic fibrosis aged 18 to 34 years. Each patient received three different drug doses (500 mg, 750 mg, and 1,000 mg) at successive one-week intervals. Dosing and drug assays were double blinded. Blood and urine were assayed over the 48 hours following each dose. Ciprofloxacin was absorbed from the gastrointestinal tract. Peak serum concentrations averaged 2.8, 4.5, and 4.6 micrograms/mL respectively at the three doses, well above the mean inhibitory concentrations of most isolates of Pseudomonas aeruginosa. Time to peak concentration was approximately two hours. The range of sputum levels in three patients was 1.1-2.1 micrograms/mL at four hours after the three doses. The serum elimination half-life was 3.7 hours and was independent of dose. Urinary recovery was 26%; greater than 90% of urinary excretion occurred within the first 12 hours. The results of this study indicate that ciprofloxacin has potential for use in the treatment of P aeruginosa infections in patients with cystic fibrosis.