Inherited thrombophilia increases the likelihood of venous thrombosis. The highest risk is associated with severe inherited thrombophilia which, if diagnosed, will often affect the duration of treatment in patients with venous thrombosis. High-risk thrombophilia is associated with antithrombin deficiency homozygosity for FV Leiden or prothrombin G20210A mutation simultaneous heterozygosity for several gene defects (double heterozygosity) protein C, and possibly protein S, deficiency. Solitary heterozygous FV Leiden or prothrombin mutation will increase the thrombotic risk only slightly. The association between inherited thrombophilia and arterial occlusion is less clearly demonstrated. In practice, arterial obstruction should only raise a suspicion of inherited thrombophilia if encountered in a young person with no known risk factors. Predisposition to thrombotic events may also be caused by acquired disturbances in the coagulation system, the most important of which is the presence of circulating phospholipid antibodies.