Svedberg N, Sundström J, James S, et al. - In a cohort of endurance trained athletes, researchers assessed the incidence of atrial fibrillation and stroke in correlation with endurance training, as well as possible sex differences associated with these links in this population. Participants were Swedish skiers (208,654) in Vasaloppet and a matched sample (n = 527,448) of nonskiers. Findings revealed a lower incidence of atrial fibrillation and stroke among female skiers in Vasaloppet vs female non-skiers. A comparable incidence of atrial fibrillation and lower risk of stroke was reported in male skiers vs non-skiers. The reported incidence of atrial fibrillation was highest among men with a higher number of races and faster finishing times. Following the diagnosis of atrial fibrillation, a lower incidence of stroke and death was observed among skiers vs nonskiers with atrial fibrillation. This implies a link between atrial fibrillation and a higher incidence of stroke in well-trained individuals, on an individual level. However, a low risk of stroke was reported on the population level and exercise should not be avoided.