The presence of arrhythmias in athletes is not infrequent. Bradyarrhythmias are more frequent in sportsmen than in the general population. This fact is often due to a 'relative vagal hypertony', owing to a training effect. Tachyarrhythmias are also present in sportsmen in almost the same percentage as in sedentary people. Any of several types of tachyarrhythmia can be seen. Abnormal heart conditions can be demonstrated in many athletes with symptomatic tachyarrhythmias. These arrhythmias frequently occur during exercise and in the early phase of recovery. An organic disorder was present in 26 of 32 athletes who were investigated with an exercise test, Holter monitoring and an electrophysiological test because of the presence of tachyarrhythmias. In six cases we were not able to find any cardiac disease. In 75% of these cases the arrhythmias had a strong relation with exercise. During follow-up (1-5 years; mean 2.88 years) some arrhythmias recurred in spite of medical treatment, in 12 of the 32 subjects, while the other 20 were free of symptoms and arrhythmias. In our opinion it is not possible to ascribe to sport, per se, the direct responsibility for these arrhythmias.