Thrombophilias may be inherited or acquired, continuing or transient, and may contribute strongly or weakly to thrombosis. They may predispose to venous thromboembolism alone or also to artery occlusion. Advice on management must recognize these variations. The presence of an inherited thrombophilia should not alter the intensity of anticoagulant therapy, given that antithrombin, protein C, or protein S deficiency, factor V Leiden, and the prothrombin G20210A mutation are not unusually anticoagulant resistant. However, they can increase the optimal treatment duration after a first thromboembolic event. Optimal duration depends on the balance between thrombosis risk off treatment and bleeding risk during extended anticoagulant therapy, and needs to be separately estimated for each individual with thrombosis and thrombophilia. The higher the thrombosis risk and the lower the bleeding risk, the longer the optimal treatment duration. This balance favors continued (but perhaps not indefinite) therapy in antithrombin, protein C, and protein S deficiency, and perhaps also in patients with the factor V Leiden or prothrombin mutations if their bleeding risk is low. Thrombosis that complicates active malignancy, the antiphospholipid syndrome, or heparin-induced thrombocytopenia needs special consideration: recent clinical trials suggest that low molecular weight heparins are more effective than warfarin in thrombosis with cancer, and that a more intense warfarin effect is not needed for patients with antiphospholipid syndrome and thrombosis. Debate continues about the place of screening for presymptomatic but affected relatives of patients with thrombosis and an inherited predisposition. It is essential that any family testing be done only with the informed consent of all concerned. Given consent, there is general support for family testing in antithrombin, protein C, or protein S deficiency and where the factor V Leiden or prothrombin mutation is strongly penetrant and expressed. There is, however, a strong argument that any testing in families in which clotting factor polymorphisms are weakly expressed should be restricted to young women when they consider hormonal contraception or pregnancy, given that these acquired factors multiply the risk.