For patients who are unsuitable for or choose not to take an oral anticoagulant (for reasons other than concerns Revision accepted August 31, 2011. Af liations: From the Department of Medicine (Drs You, Schulman, and Spencer) and Department of Clinical Epidemi- ology and Biostatistics (Dr You), McMaster University, Hamilton, ON, Canada; Department of Medicine (Dr Singer), Harvard Medical School, and Clinical Epidemiology Unit (Dr Singer), General Medicine Division, Massachusetts General Hospital, Boston, MA; School of Pharmacy (Dr Howard), University of Kansas Medical Center, Kansas City, KS; University of Birmingham Centre for Cardiovascular Sciences (Drs Lane and Lip), City Hospital, Birmingham, England; Department of Clinical Medi- cine (Dr Eckman), Division of General Internal Medicine and Center for Clinical Effectiveness, University of Cincinnati, Cin- cinnati, OH; Department of Medicine (Dr Fang), Division of Hospital Medicine, University of California, San Francisco, San Francisco, CA; Boston University Medical Center Research Unit (Dr Hylek), Section of General Internal Medicine, Boston, MA; Comprehensive Clinical Research Unit (Dr Go), Division of Research, Kaiser Permanente Northern California, Oakland, CA; Decision Resources Inc (Dr Hughes), London, England; Sec- tion of Non-invasive Cardiac Imaging (Dr Manning), Beth Israel Deaconess Medical Center, Boston, MA; and The Cardiovas- cular Institute (Dr Halperin), Mount Sinai Medical Center, New York, NY. Funding/Support: The Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines received support from the National Heart, Lung, and Blood Institute [R13 HL104758] and Bayer Schering Pharma AG. Support in the form of educa- tional grants was also provided by Bristol-Myers Squibb; P zer, Inc; Canyon Pharmaceuticals; and sano -aventis US. 