Venous thromboembolism (VTE) comprises deep vein thrombosis (DVT) with or without symptomatic pulmonary embolus (PE). The incidence of a first episode of VTE is 1.5 per 1000 person-years [1] (J Thromb Haemost, 2007;5:692-9) with a per-person lifetime incidence of 5% [2] (Arch Intern Med 1998;158:585-93). The risk of recurrence after DVT and PE is similar but the pattern of recurrence tends to reflect the initial event, for example recurrence with PE is more common in patients with previous PE [3] (Circulation 2003;107:122-30). At least 50% of patients, who present with symptomatic DVT, have asymptomatic PE and conversely, a majority presenting with symptomatic PE have asymptomatic DVT [3] (Circulation 2003;107:122-30). This suggests that whilst DVT and PE are manifestations of the same pathology, the phenotypic expression of the disease is predetermined. This may be an important consideration for long-term anticoagulant therapy as the risk of fatal PE is the greatest in patients with previous PE [4] (Ann Intern Med 2007;147:766-74). At present, the only factor reported to be associated with the pattern of VTE is the factor V Leiden mutation [5] (Thromb Haemost 1999;81:345-8). This suggests that the kinetics of thrombin generation and the resulting fibrinolytic response may influence clot structure and likelihood of embolization.