Coordination of the study: The study will be coordinated by the Thromboembolism Research Unit at St Joseph's Hospital in Hamilton, Ontario. Data submission to the coordinating centre will occur over a secure, web-based database system, which will be supplemented by paper copies of the case report forms which will be maintained at each clinical centre. Details of the web-based data-entry system are currently being determined; a revised study manual with full data entry details will be included in the subsequent versions of this study manual.