The clinical situation for patients receiving extracorporeal membrane oxygenation (ECMO) is complex, and drug dosing is complicated by significant pharmacokinetic alterations. We sought to describe the frequency of achievement of therapeutic vancomycin concentrations in critically ill patients receiving ECMO with therapeutic drug monitoring (TDM). In this retrospective observational study, we included all critically ill patients receiving TDM for vancomycin while on ECMO. The primary outcome was the proportion of plasma vancomycin concentrations in the therapeutic range (15 to 20 mg/liter).