The aim of this study was to evaluate the effect of augmented renal clearance (ARC) on vancomycin serum concentrations in critically ill patients. This prospective, single-centre, observational, cohort study included 93 consecutive, critically ill septic patients who started treatment that included vancomycin by continuous infusion, admitted over a 2-year period (March 2006 to February 2008). ARC was defined as 24-h creatinine clearance (CL Cr )>130mL/min/1.73m 2 . Two groups were analysed: Group A, 56 patients with a CL Cr ≤130mL/min/1.73m 2 ; and Group B, 37 patients with a CL Cr >130mL/min/1.73m 2 . Vancomycin therapeutic levels were assessed on the first 3 days of treatment (D 1 , D 2 and D 3 ). Serum vancomycin levels on D 1 , D 2 and D 3 , respectively, were 13.1, 16.6 and 18.6μmol/L for Group A and 9.7, 11.7 and 13.8μmol/L for Group B (P<0.05 per day). The correlation between CL Cr and serum vancomycin on D 1 was −0.57 (P<0.001). ARC was strongly associated with subtherapeutic vancomycin serum concentrations on the first 3 days of treatment.