Venous thromboembolism is a major national health problem, with at least 201,000 first lifetime cases per year in the United States. Over 50,000 of these patients die within seven days, and for 20 percent, death is rapid, with insufficient time for medical intervention. Recurrent VTE is frequent and also associated with death as well as chronic impairment due to the venous stasis syndrome. Thus, identification of high-risk patients for targeted VTE prophylaxis or treatment is necessary to improve survival and prevent impairment. While clinical risk factors can identify a population at risk, such risk factors have low predictive value for the individual patient. The high prevalence of the Factor V R506Q (Leiden) and Prothrombin 20210G --> A mutations among VTE patients suggests the hypothesis that additional genetic abnormalities of coagulation (thrombophilia) are associated with a substantial proportion of VTE cases.