The pharmacokinetics of cefoxitin was studied in patients with renal impairment during haemofiltration and in the intervening periods after administration of 30 and 15 mg/kg of the drug, respectively. Different pharmacokinetic patterns were established during haemofiltration and in the interim period, with average elimination half-lives of 11.85 +/- 4.3 and 3.41 +/- 0.6 h, respectively. The average fraction of the cefoxitin dose eliminated in haemofiltration was 0.62 +/- 0.11, more than that established in haemodialysis. In patients with terminal renal impairment undergoing haemofiltration every 48 h, a dose of 15 or 30 mg/kg is recommended at the start and at the end of each haemofiltration session.