Clinical practice and the results of some studies may indicate that physical exercise in the form of endurance training may influence the development of atrial fibrillation (AF). The aim of this paper is to evaluate the scientific background for the hypothesis that there is a connection between physical activity and AF.This paper is a review article based on searches in PubMed on specific topics, limited to the period 1995 through March 2011. We found 17 original articles and three relatively recent reviews. Each was read by at least two of the authors and then discussed. Seven of the original articles were excluded for methodological reasons, and we therefore discuss the other ten.We found support for the hypothesis that systematic high intensity endurance training such as running can increase the risk of AF, whereas the studies provide no evidence that less intensive physical exercise such as walking increases the risk. Several of the studies have methodological weaknesses.Important questions remain unanswered. There is a need for more studies that can shed light on the connection between training intensity, total volume of intensive endurance training, age-related changes and AF. Studies that include women are also needed.