Abstract
Patients with venous thromboembolism can be divided into two groups. The first includes patients with a disease such as cancer, a predisposing factor such as recent surgery, or an acquired abnormality such as the lupus anticoagulant that is known to increase the risk of thrombosis. The mechanisms underlying the hypercoagulable state in this group are poorly understood. The second category consists of patients without the usual risk factors that predispose people to venous thrombosis. In some of these patients it is possible to identify a deficiency of antithrombin III, protein C, or protein S. Family studies have shown that hereditary . . .