Ventricular cerebrospinal fluid (CSF) trimethoprim-sulphamethoxazole (TMP-SMZ) concentrations were measured in 15 patients. At varying times, not exceeding eight hours prior to surgery, each patient received a dose of 5 mg/kg TMP with 25 mg/kg SMZ as a 1 h intravenous infusion. The mean ventricular CSF TMP concentration was 1.1 mg/l with a range of 0.5-3.2 mg/l. The SMZ concentration was 17.9 mg/l with a range of 5-40 mg/l. There was no apparent relationship between achievable CSF concentrations and the time elapsed after drug administration. The CSF concentrations were achieved despite the lack of meningeal inflammation. In 11 of 14 patients, the levels exceeded the minimum inhibitory concentrations for Staphylococcus aureus and Staph. epidermidis.