METHODS In this multi centre pilot study we will perform TEE in patients with AF who are eligible for VKA treatment. TTE will be used as pre-screening: if TTE shows left atrial abnormalities or aortic plaque, patients will be excluded from randomisation. TEE will be performed in all other patients to detect or exclude complex aorta plaques or signs of left atrial stasis. Three hundred patients who do not have these features on TTE will be randomly assigned to treatment with aspirin or VKA. Follow-up will be 1 year.