To evaluate the impact of dabigatran on ADP induced platelet aggregation we will randomize patients with atrial fibrillation and the need for oral anticoagulation for a two-week treatment with either dabigatran or phenprocoumon and we hypothesize that dabigatran is superior to phenprocoumon in the reduction of ADP induced platelet aggregation. Patients who are concomitantly treated with clopidogrel are being studied in a different trial with a similar study design (Dabi ADP-2).