Colistin pharmacokinetics were prospectively studied after intravenous administration of colistin methanesulphonate in critically ill patients without central nervous system infection (controls, n = 5) and in patients with external ventricular drain-associated ventriculitis after intravenous administration (EVDViv, n = 3) or combined intravenous/intraventricular administration (EVDVcomb, n = 4). Cerebrospinal fluid (CSF)/serum colistin concentration ratios were higher in EVDViv than in control patients (11% versus 7%, P ≤ 0.05) and in EVDVcomb compared to all other patients (P < 0.0001). CSF colistin concentrations above the MIC of 0.5 μg/ml were achieved only in EVDVcomb patients.