Inclusion Criteria:

• Documented persistent atrial fibrillation
• Age of 18 years
• Documented oral anticoagulation (INR ≥ 2) for at least three weeks prior to inclusion, or exclusion of left atrial thrombi by trans-esophageal echocardiography
• Written informed consent of the patient

Exclusion Criteria:

• Current therapy with antiarrhythmic agents of class I and class III other than study medication flecainide. Such antiarrhythmic treatment must be stopped five half lives prior to enrollment. Five half lives correspond to 48 hours for almost all antiarrhythmic agents. For details regarding a specific agent, this information can be obtained through the internet at www.rote-liste.de or from the Fachinformation of the specific compound.
• Long-term therapy with amiodarone within the last 6 months prior to inclusion
• Symptomatic bradycardia or symptomatic sick sinus syndrome unless treated with a permanent pacemaker
• Symptomatic higher degree AV nodal block (grade II or III) unless treated with a permanent pacemaker
• Brugada syndrome
• Typical angina pectoris symptoms at rest or during exercise
• Known untreated coronary artery disease with high-degree coronary stenosis (> 80% reduction in luminal diameter)
• Myocardial infarction within the last 3 months
• Left ventricular ejection fraction  40%
• Creatinine clearance < 50 ml/min*1.73 m2 as determined by the Cockroft-Gould formula. The digital data management system will calculate this value for you during the inclusion process. For completeness of documentation, the formula is given below:
• Men: Creatinine clearance (ml/min) = (140 – age(years)) * body weight (kg) / (72 * serum creatinine level (mg/dl))
• Women: Value for men * 0,85
• Manifest hepatic insufficiency
• Hyperthyroidism or hypothyroidism manifested clinically and in laboratory tests (TSH, T3, T4)
• Females who are pregnant or breast feeding
• Females of childbearing potential who are not using a scientifically accepted method of contraception
• Participation in a clinical trial within the last 30 days. Simultaneous participation in a registry (e.g. project AB1 of the AFNET) is permitted.
• Drug addiction or chronic alcohol abuse
• Legal incapacity, or other circumstances which would prevent the patient from understanding the aim, nature or extent of the clinical trial
• Evidence of an uncooperative attitude
• Prolongation of the QRS complex by more than 25% during flecainide treatment (measured as the difference in QRS duration between the baseline ECG and the ECG at cardioversion (34))