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To define the steady-state pharmacokinetics of colistin methanesulphonate and colistin in patients with cystic fibrosis (CF) following intravenous administration of the former.The study was conducted in 12 patients with CF following intravenous administration of colistin methanesulphonate (1.63–3.11 mg/kg) every 8 h for at least 2 days. On the day of study, four blood samples were collected from each patient at 60, 120, 240 and 360 min after the end of the infusion. Concentrations of colistin methanesulphonate and colistin in plasma were measured separately by HPLC.At steady-state, colistin methanesulphonate had a mean (± S.D.) total body clearance, volume of distribution and half-life of 2.01 ± 0.46 mL/min per kg, 340 ± 95 mL/kg and 124 ± 52 min, respectively. Colistin had a significantly longer mean half-life of 251 ± 79 min (P < 0.001). With the regimen used, colistin methanesulphonate was well tolerated. This is the first report on the pharmacokinetics of colistin methanesulphonate in CF patients determined using concentrations of colistin methanesulphonate and colistin in plasma.Based on the in vitro pharmacodynamics against Pseudomonas aeruginosa previously published by our group and these pharmacokinetic findings, dose escalating trials may be warranted to maximize efficacy.