The inhibition of the main natural anticoagulant antithrombin, which acts by inactivating activated factor X (FX) and thrombin, is an interesting therapeutic approach in the frame of the upcoming drugs for hemophilia A and B. Data on the influence of natural anticoagulant levels on the bleeding phenotype of patients with hemophilia was first reported in the literature more than ten years ago in a study demonstrating the association between thrombophilia abnormalities and a milder phenotype in patients with severe hemophilia A [13]. This evidence concerns the gene F10 and hemophilia.