There are many established risk factors for atrial fibrillation (AF): in addition to age, the strongest risk factors reported from a cohort study in Malmø, Sweden, were a history of heart failure or myocardial infarction, whereas the largest population-attributable risks were observed for hypertension and obesity.1 Thus, the majority of AF cases are related to established cardiac diseases or to the presence of cardiovascular risk factors. The lack of physical exercise is now included in the list of major cardiovascular risk factors. Physical activity should then, in a dose-dependent manner, be expected to reduce the incidence of AF. However, increasing evidence suggests that excessive physical exercise may be harmful, and that the dose–relationship between physical exercise and cardiovascular health may be U-shaped rather than linear for death from cardiovascular causes. A U-shaped curve may also exist for AF; several reports have suggested that also AF may be related to excessive endurance exercise, and physical exercise has been included among the risk factors for AF in the 2014 update of the American Heart Association/European Society of Cardiology/Heart Rhythm Society (AHA/ESC/HRS) AF guidelines.2 The majority of the studies that have examined the relationship between physical activity and AF has been limited to men, and in particular, male endurance athletes seem to have a rather high prevalence of AF. It is therefore pertinent that Drca et al3 now provide data from a large Swedish female cohort recruited from the general population, to clarify this issue …