Introduction: Data on the optimal dosage of cefotaxime (CTX) in patients receiving continuous renal replacement therapy (CRRT) is sparse and equivocal [1]. We conducted a trial investigating the concentrations of CTX in general ICU and post-cardiac surgery ICU patients who were treated with CRRT because of acute renal failure and who received CTX 4D 1 g iv. as part of selective gut decontamination. Methods: Twenty seven patients (16 postcardiac surgery and 11 general ICU) were included who received at least 48 h concomitant CRRT and CTX treatment. Plasma concentrations of CTX and its active metabolite desacetylcefotaxime (DAC) were measured in all plasma samples left over from routine laboratory tests. Pharmacokinetic values (CTX peak and trough levels) were calculated using MWPharm v3.70 (Medi Ware, Groningen, The Netherlands) Results: The results showed comparable peak and trough levels of CTX in both postcardiac surgery and general ICU patient groups (Table 1). Peak CTX levels were within normal range with wide inter-individual variation. In all patients except one, CTX levels were above the minimum inhibitory concentration (MIC) (4 mg . L-1, Enterobacteriaceae) > 90% of the time. Five patients showed through levels below the MIC. DAC concentrations were comparable in both patient groups and corresponding with literature (median 15 mg . L-1, range 2-38 mg . L-1) and thus assumed to contribute to the effectiveness of the treatment. Discussion: A regime of 4D 1 g CTX iv. leads to adequate concentrations in the majority of patients. Dose reduction to 2D 1 g CTX iv. as proposed by others, might lead to sub-therapeutic levels in at least 20% of these patients.