Role of the participating investigators: Dr Mark Crowther is the principal applicant and will be primarily responsible for all aspects of the study. Drs Ageno and Garcia, in addition to fulfilling the roles of co-investigators, will coordinate recruitment and management of centres in Italy, and the United States, respectively. Drs Crowther, Ageno and Garcia will comprise the executive steering committee which will meet by teleconference at least monthly to discuss specific trial implementation and management issues. The steering committee will consist of the members of the executive steering committee, as well as the Canadian participating investigators (Drs Blostein, Anderson, Wells, Kahn, Ginsberg, Kovacs, Kearon and Selby) and Dr Witt from the United States. Telephone Steering Committee meetings will occur six monthly, or more frequently as determined by the Principal Investigator. The Steering Committee is responsible for the overall conduct of the study, and will act on the recommendations of the study's subcommittees.