Inclusion criteria Patients with non-valvular atrial fibrillation (AF), at moderate to high risk of stroke, or systemic embolism with at least one additional risk factor (i.e. previous ischemic stroke, TIA, or systemic embolism, left ventricular dysfunction, age >=75 years, age >=65 with either diabetes mellitus, history of coronary artery disease or hypertension)

Exclusion criteria

1. Prosthetic heart valves requiring anticoagulation per se, or hemodynamically relevant valve disease that is expected to require surgical intervention during the course of the study
2. Severe, disabling stroke within the previous 6 months, or any stroke within the previous 14 days
3. Conditions associated with an increased risk of bleeding
4. Contraindication to warfarin treatment
5. Reversible causes of atrial fibrillation (e.g., cardiac surgery, pulmonary embolism, untreated hyperthyroidism).
6. Plan to perform a pulmonary vein ablation or surgery for cure of the AF
7. Severe renal impairment (estimated creatinine clearance <=30 mL/min)
8. Active infective endocarditis
9. Active liver disease
10. Women who are pregnant, lactating, or of childbearing potential who refuse to use a medically acceptable form of contraception throughout the study
11. Anaemia (haemoglobin <100g/L) or thrombocytopenia (platelet count <100 x 109/L)
12. Patients who have developed transaminase elevations upon exposure to ximelagatran
13. Patients who have received an investigational drug in the past 30 days or are participating in another drug study
14. Patients considered unreliable by the investigator concerning the requirements for follow-up during the study and/or compliance with study drug administration, has a life expectancy less than the expected duration of the trial due to concomitant disease, or has any condition which in the opinion of the investigator, would not allow safe participation in the study (e.g., drug addiction, alcohol abuse)
15. Any known hypersensitivity to galactose if the warfarin used contains galactose.