During the last years there has been an increased media focus on arrhythmias in previously healthy athletes. This assignment sought to investigate the relationship between development of atrial fibrillation and endurance-sports. In 2010, Grimsmo et al. published The BIRKOPP -study, a 28-30 years follow-up of long-term endurance cross-country skiers, where they found a high prevalence (12,8%) of LAF (lone atrial fibrillation). Several studies confirm the high prevalence of LAF in people engaged in endurance-sports, but potential predictors and mechanisms for developing LAF are still not fully clear. Some of the literature describes low resting heart rate, long PQ- time and dilatation of left atrium as factors associated with LAF and endurance- sports. There are few studies in which the relationship between training intensity and accumulated training hours are recognized as relevant for developing LAF. The studies on the topic of endurance-sports and LAF also often tend to have small populations, mainly consisting of men. Future studies should be multiple, have larger populations and also include more women. With this, one can hopefully draw more sound conclusions on the relationship between atrial fibrillation and endurance-sports.