Bakground and study rationale: Left atrial (LA) thrombi are found in 2.5 - 18% of subjects with atrial fibrillation (AF) depending on the subject population. Subjects with LA thrombi have a particularly increased risk for cerebral and peripheral embolism. Consequently, oral anticoagulation is recommended in those subjects. Transesophageal echocardiography (TEE) is suited and is capable of visualizing and deecting LA thrombi. Cranial magnetic resonance imaging (MRI) has a high accurancy for detecting ailent cerebral mircoembolism. However, under continued effective anticoagulation with the vitamin K antagonist (VKA) Phenprocoumon we have shown that only 56% of LA thrombi resolve during a 12 months TEE observational period. At 1 month only 16% of the thrombi disappeared. However, cureent treatment guidelines recommend 4 weeks of effective anticoagulations prior cardioversion, independent of LA thrombus. In our subject population 84% of LA thrombi would have still been presented at 4 weeks VKA therapy and thus, would have had an increased risk of thromboembolism, if cardioversion, in accordance with current guidelines, would have performed. It is well known that thrombin plays a central role in the formation, growth, maintenance, and consolidation of thrombus. Direct thrombin inhibition has been shown to block these processes and leads to inhibition of thrombus. In vivo, it has been shown to reduce 90% of the preformed, half-hour-old-thrombus. This effect is probably due to better inhibition of the catalytically active clot-bound thrombin. Newer oral anticoagulants, like the direct thrombin inhibitor Dabigatran are therefore very promising for resolution of LA thrombi in comparison to VKA, which has not been investigated previously.