Overview: This is a single-center, randomized, two treatment groups, open, phase IV clinical study in subjects with atrial fibrillation and left atrial thrombus. At visit 0 (Screening visit) subjects with left atrial thrombus established in TEE are eligable to entry in the study. If the subjects fulfill all other in- and exclusion criteria and undersign the informed consent the baseline cranial MRI can be performed follwoed by randomization to one of the two treatment groups (Phenprocoumon or Dabigatran) at Visit 1 (Baseline visit). The subjects will be treated for 12 months with Phenprocoumon (INR adjusted once daily) or Dabigatran 150 mg twice daily. Routinly clinical follow up visits bill be done at week 4, month 3, month 6, month 9 and month 12. Follow up TEE will be performed after week 4 and month 12 also the follow up cranial MRI at this visit. If the subject was randomized in the VKA group routinely INR measurements will performed.