The risk of venous thromboembolism determined by the presence of a single inherited thrombophilic alteration, such as factor V Leiden or prothrombin mutation, is difficult to estimate. Nevertheless, the coexistence of these disorders with a blood group different than O considerably increases the risk of venous thromboembolism in the general population. Since the analysis of the blood group can be easily performed, in all patients with other risk factors for venous thromboembolism particular attention should be given to the presence of blood groups different than O.