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Thirteen patients with cystic fibrosis and 12 healthy control volunteers received a single oral 800 mg dose of fleroxacin and 800 mg every day for 5 days. Interstitial fluid penetration was studied by the suction-induced blister technique. Fleroxacin and its two major metabolites, N-demethyl and N-oxide, were analyzed in plasma and urine by HPLC. Single-dose absorption parameters (absorption rate constant, normalized peak plasma drug concentration, and time to reach peak concentration) and total urinary excretion indicated that fleroxacin was absorbed more slowly and more completely in patients with cystic fibrosis than in control subjects. Fleroxacin volume of distribution tended to be smaller in patients with cystic fibrosis and it reached statistical significance after a single dose when normalization for lean body mass was applied. When normalized for lean body mass, the formation clearance of N-demethyl fleroxacin and N-oxide fleroxacin was significantly greater in patients with cystic fibrosis than in control subjects (p less than 0.05). These data concur with those of others showing an induction of drug-metabolizing enzymes in cystic fibrosis. Renal clearances of fleroxacin and its metabolites were significantly increased in cystic fibrosis (p less than 0.05), and this seems to be explained by a decreased tubular reabsorption of these compounds. The differences seen in the pharmacokinetics of fleroxacin in cystic fibrosis support the theories of generalized induction of drug metabolism and of a defective renal tubular reabsorptive process of drugs in cystic fibrosis.