See also Rosendaal FR. Etiology of venous thrombosis: the need for small original studies. This issue, pp 2189–90 and El-Galaly TC, Kristensen SR, Overvad K, Steffensen R, Tjønneland A, Severinsen MT. Interaction between blood type, smoking and factor V Leiden mutation and risk of venous thromboembolism: a Danish case–cohort study. This issue, pp 2191–3.