Inclusion Criteria:

• ECG-documented atrial fibrillation (AF) and an indication for long-term vitamin K antagonist (VKA) therapy based on the presence of at least one of the following risk factors:

  1. previous ischemic stroke, transient ischemic attack (TIA) or systemic embolism
  2. hypertension requiring drug treatment
  3. left ventricular dysfunction (left ventricular ejection fraction <45% or symptomatic congestive heart failure)
  4. age >75 years
  5. age between 65-75 years plus diabetes mellitus, or
  6. age between 65-75 years plus symptomatic coronary artery disease (previous myocardial infarction (MI) or angina pectoris)
• Written informed consent

Exclusion Criteria:

• Legal lower age limitations (country specific)
• Indication for VKA other than AF, including prosthetic heart valves, venous thromboembolism, and planned cardioversion
• Transient AF caused by a reversible disorder
• Active bleeding or high risk of bleeding
• Recent (<15 days) or anticipated invasive procedures with potential for uncontrolled bleeding, including major surgery
• Participation in another pharmacotherapeutic study within the prior 30 days
• Creatinine clearance <10 mL/min, severe hepatic disease, or bacterial endocarditis
• Uncontrolled hypertension: systolic blood pressure > 180 mm Hg and/or diastolic blood pressure > 110 mm Hg
• Pregnancy or childbearing potential without proper contraceptive measures
• Breastfeeding
• Any other contraindication listed in the labeling of warfarin or acenocoumarol