ATI-5923 is a structural analog of warfarin that is being developed as an oral anticoagulant. It has a similar pharmacological profile to warfarin but was designed to provide more uniform and stable anticoagulation. ATI-5923 is a selective, non-competitive inhibitor of vitamin K epoxide reductase. ATI-5923 is metabolized by esterase to a single inactive metabolite and thus is expected to have less variable metabolism, drug-drug interactions and reduced instances of under/over coagulation, making the response safer and more predictable. The primary purpose of this study is to develop a dosing regimen and INR monitoring schedule that can be applied in an outpatient setting to safely and conveniently anticoagulate patients with atrial fibrillation to a therapeutic INR. Patients with atrial fibrillation (AF) were chosen for the study population of this trial because they require anticoagulation if they have risk factors for stroke.