Some people have coagulation abnormalities, collectively referred to as thrombophilia, which increase the risk of thrombosis. What are the most frequently detected thrombophilia? Does throm- bophilia testing after a venous thromboembolic event enable effective adjustment of the treatment strategy?To answer these questions, we reviewed the available evidence using the standard Prescrire methodology. The best known inherited thrombophilia includes the factorV Leiden mutation and the prothrombin G20210A mutation. Hereditary deficiency of the anticoagulants protein C, protein S and antithrom- bin are rarer, and less is known about them. Inher- ited thrombophilia increases the risk of venous thromboembolism to varying degrees compared with the general population, but they have not been shown to increase the risk of arterial thrombosis. The most common acquired thrombophilia is the presence of antiphospholipid antibodies.They can occur alone or in conjunction with autoimmune diseases such as systemic lupus erythematosus. Patients with antiphospholipid antibodies are at increased risk of both venous and arterial thrombosis. Venous thromboembolism usually occurs after a precipitating event and in the presence of risk factors. Thrombophilia is just one of the risk factors for venous thrombosis. Venous thrombo- embolism at a young age in a first-degree relative is another risk factor, even in the absence of a detected inherited thrombophilia. No comparative randomised clinical trials have explored the value of thrombophilia testing in helping to make informed treatment decisions following venous thromboembolism. The known thrombophilias do not affect the efficacy of anticoagulants. Knowing that a patient has a thrombophilia has no impact on the choice or dose of anticoagulant. In cases of unprovoked venous thromboembol- ism, the known inherited thrombophilia do not appear to have a tangible impact on the risk of recurrence after discontinuation of anticoagulation. The increased risk of recurrence associated with the presence of antiphospholipid antibodies appears greater than the risk associated with inherited thrombophilia. The estimated magnitude of this increased risk varies across studies. Clinical guidelines only suggest performing thrombophilia testing after a venous thromboem- bolic event in certain situations, for patients with no identified risk factors for recurrence, when the result might influence the decision to continue or stop anticoagulation: testing for inherited thrombophilia if a close relative had unexplained venous thrombosis at a young age, and antiphospholipid antibody testing. The identification of a thrombophilia can lead to overestimating the risk of thrombosis, and underestimating the risk of bleeding in patients receiving anticoagulation. In practice, thrombophilia testing is rarely useful following venous thromboembolism, except perhaps to clarify the risk of recurrence in some patients in whom the thromboembolic event was unexplained, when deciding whether to discon- tinue anticoagulation.