Objectives: To evaluate the pharmacokinetics of polymyxin B in patients on continuous venovenous haemodialysis (CVVHD) after intravenous administration of unadjusted dosage regimens. Patients and methods: Two critically ill patients had eight blood samples collected during a 12 h interval on days 8 and 10 of polymyxin B therapy. Dialysate was collected every hour during the 12 h dosing interval. Polymyxin B binding in plasma was determined by rapid equilibrium dialysis. Concentrations of polymyxin B in plasma and dialysate samples were quantified using a validated ultra-performance liquid chromatography-tandem mass spectrometry assay. Results: Respective maximum plasma concentrations in patients 1 and 2 were 8.62 and 4.38 mg/L; total body clearances (scaled linearly by body weight) were 0.043 and 0.027 L/h/kg, respectively, of which 12.2 and 5.62 were dialysis clearance, respectively. The corresponding volumes of distribution of polymyxin B at steady state were 0.50 and 0.34 L/kg, respectively, and protein binding in pooled plasma samples was 74.1 and 48.8 , respectively. Conclusions: Our findings indicate that the recommended polymyxin B doses should not be reduced for patients on CVVHD.