Atrial fibrillation (AF) is the most common arrhythmia in clinical practice, affecting approximately 1-3% of the general population in Switzerland. Despite the available evidence on the association of AF with stroke and systemic embolism, the underlying mechanisms of thrombogenesis with regard to rhythm and/or other factors are largely unknown. If the presence of AF per se is the main reason for thrombus formation, more AF episodes should result in more strokes. There is at least some evidence that non-paroxysmal AF is associated with a higher risk of stroke. However, the temporal relationship of AF episodes with the occurrence of strokes has been questioned. Therefore, the significance of AF burden, defined as the percentage of time in AF and its changes over time are currently largely unknown.Aims: The aim of this study is to increase our knowledge on the association between AF burden and its health consequences, mainly stroke/systemic embolism and cognitive dysfunction. Methodology: Patients from the SNSF funded Swiss-AF cohort study with paroxysmal and persistent AF will be asked to participate in the study. Based on 1) an enrollment rate of 40% of all Swiss-AF patients with paroxysmal and persistent AF (according to a pilot study), and 2) the participation of the top enrolling centers, it is expected to enroll 460 patients. All patients will get a 7-day Holter electrocardiogram (ECG), in order to assess the AF burden. The 7-day Holter ECG will be repeated after one year. A subsample of 100 patients will additionally get an implantable loop recorder to monitor their cardiac rhythm continuously over two years. Measurement of AF burden will be performed using an automated AF episode detection algorithm and manual validation. In addition to the cardiac monitoring, these patients will get a cardiac magnetic resonance imaging to assess cardiac structure and function at baseline. All other characteristics (clinical information, blood samples, brain MRI) and follow-up information are available within the Swiss-AF cohort. Potential Significance: This multicenter interdisciplinary project will provide novel and unique insights on the association of directly measured AF burden and its consequences. Whereas the infrastructure cohort grant Swiss-AF covers important variables including validated outcome events and an established biobank in a large number of AF patients, the association of AF with stroke and other brain lesions is currently limited by the fact that AF burden is not quantified and that information on cardiac function is not assessed systematically in our patient population. The added information of measuring AF burden and obtaining structural cardiac information in an AF patient population where brain MRIs are available, will provide further evidence towards a better understanding of the correlation between AF and stroke.