To study the course and circumstances of atrial fibrillation in endurance sports. Atrial fibrillation is the main cause of symptoms in athletes and frequently disturbs the performance during the practising of sport. The course of the arrhythmia is seldom reported in this population. In 1993 and 2002 symptoms and clinical presentation of atrial fibrillation were evaluated in 30 well-trained athletes with a specially designed questionnaire. In 1993 paroxysmal atrial fibrillation was present in 30 male athletes at the mean age of 48.1±7.8, 3 (10%) of them also had paroxysmal atrial flutter. Three (10%) of the athletes died before 2002. In 2002 paroxysmal atrial fibrillation continued in 15 (50%) athletes, permanent atrial fibrillation emerged in 5 (17%) athletes and 7 (23%) of them showed no further atrial fibrillation. In 1993 paroxysms of atrial fibrillation started at a relatively low level of training intensity compared with the mean maximal training intensity of 11±7 versus 8±4 h/week (ppp<0.05). In 10 (38%) of the athletes a familiar form of paroxysmal atrial fibrillation was present. A small proportion of the athletes (26%) was asymptomatic. Paroxysmal atrial fibrillation remained stable in half of the athletes whereas the arrhythmia changed into permanent atrial fibrillation in a minority of this population.