Evaluation of inherited hypercoagulability in patients with venous thromboembolism includes testing for the functional activity of protein S, protein C, antithrombin III, and for resistance to activated protein C. Resistance to activated protein C can be assessed with plasma as well as with DNA-based assays that are commercially available. Acquired disorders include the development of antibodies against phospholipid-protein complexes (as would occur in patients with the antiphospholipid syndrome). The inherited and acquired abnormalities are most commonly apparent as superficial or deep venous thromboembolism in younger patients, most of whom have additional risk factors for thrombosis, such as use of oral contraceptives or recent trauma. These tests are cost effective if the results will influence patient management or will have potential value in the care of family members in situations of increased thrombotic risk.