Until recently, only compassionate use of ciprofloxacin in children with cystic fibrosis was possible despite limited pharmacokinetic data. We studied five subjects with cystic fibrosis and exacerbation of pulmonary infection. A 15 mg/kg b.i.d. regimen of oral ciprofloxacin was administered. On the 15th day, eight blood samples were collected and plasma concentrations were measured by HPLC. The actual dose administered ranged from 10 to 14 mg/kg b.i.d., due to the fixed-dosage formulation. Corresponding AUC0-12 ranged from 8.2 to 11.9 mg.h/L. Plasma concentrations were maintained above individual MICs for a median time of 12.7 h over 24 h. The median area under the inhibitory curve was 52.8, which is about half the proposed target value for ciprofloxacin in pulmonary infections with Gram-negative bacteria such as Pseudomonas aeruginosa. A higher dose, administered from a specific formulation to ensure precise dosing, must be given in order to obtain adequate concentrations in cystic fibrosis children.