Extracorporeal membrane oxygenation (ECMO) is a life support for patients with severe acute respiratory failure when conventional treatments failed but it is accompanied by a significant risk of bleeding. Heparin is required during ECMO to avoid circuit thrombosis and its anticoagulant effect is strictly dependent on antithrombin (AT). AT also plays a central role in mediating inflammation. Acquired AT deficiency is common in patients on ECMO, arguably due to long term anticoagulation in addition to sepsis itself. Guidelines suggest to supplement AT in ECMO only when its deficiency coexists with heparin resistance. Few studies evaluated the effect of AT supplementation during ECMO without a consensus on the appropriate level to be maintained. AT supplementation increase anti-Factor Xa (anti-Xa) levels without increasing heparin dosage. This may have a clinical impact because risk of bleeding during ECMO is associated with higher heparin dosage.