Atrial fibrillation (AF) is one of the most frequent findings in the general population, with an incidence of about 0.1% per year and a mean prevalence of 0.95%. However, prevalence values are known to vary with age, being very low (0.1-0.2%) in subjects less than 55 years old, the age group comprising the greatest number of individuals who practice sport at various levels. Long-term, regular, intense physical training determines an increase in vagal tone, leading to bradycardia, and during physical activity, particularly competitive sport, a marked release of catecholamines occurs as a result of both the intense physical effort and the emotional stress involved in competition. Both these adaptive phenomena on the part of the autonomous nervous system may promote the development of AF. Heart diseases with a certain degree of severity are not generally compatible with sport, unless the activity undertaken involves a low expenditure of energy and is noncompetitive. In any case, in the presence of AF, the arrhythmia must be well controlled and well tolerated during performance of the activity.