Trough cerebrospinal fluid (CSF) ceftriaxone concentrations were measured daily to investigate the effect of dexamethasone on ceftriaxone penetration into CSF in adult patients with acute bacterial meningitis. Patients were divided into two groups in this double blind randomized study. In group 1 (n = 6) patients were given ceftriaxone with dexamethasone whereas in group 2 (n = 6) patients were only administered ceftriaxone. Plasma and CSF samples were collected at 24, 48, 72, 96 and 264 h following the study treatments. The trough CSF ceftriaxone concentrations were measured using high performance liquid chromatography (HPLC) and microbiological assay. CSF ceftriaxone concentrations were 3.21 mg/l at 24 h in group 1 and 4.85 mg/l at the same time in group 2 by HPLC. Although microbiological assay results were lower than HPLC the trough CSF ceftriaxone concentrations in dexamethasone group were at least 10(3) times higher than the minimum inhibitory concentrations of the susceptible strains. It was concluded that the ceftriaxone concentration in CSF was adequate and ceftriaxone penetration was not significantly affected by concomitant dexamethasone use in adult patients with acute bacterial meningitis. (C) 2003 Published by Elsevier Science B.V. and the International Society of Chemotherapy.