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 transesophageal echocardiography (TEE) are eligible to entry in the study. If the subjects fulfill all other in- and exclusion criteria and undersign the informed consent the baseline cranial magnet resonance imaging (MRI) can be performed followed 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. Routinely clinical follow up visits will be done at week 4, month 3, month 6, month 9 and month 12. Follow up TEE will be performed after 4 weeks and after 12 months also the follow up cranial MRI at this visit. If the subject was randomized in the VKA group routinely INR measurements will performed.