Atrial fibrillation (AF) is the most commonly occurring arrhythmia in clinical practice, however, the mechanism of atrial fibrillation is not well explained. It has been considered that myocardial fibrosis plays a role in atrial fibrillation. Within the heart, this fibrosis is thought to be mediated by transforming growth factor-beta 1 (TGF-beta 1), a potent stimulator of collagen-producing cardiac fibroblasts. Recent studies indicate that atrial fibrillation is associated with elevated serum concentrations of TGF-beta 1 and C-terminal propeptide of procollagen type I (a marker of collagen type I synthesis). TGF-beta 1 was not only associated with the presence of atrial fibrillation but may also predict patients at increased risk for future development of atrial fibrillation. Why TGF-beta 1 in atrial fibrillation is high is a puzzling problem. We hypothesized that TGF-beta 1 is a possible cause of atrial fibrillation by initiating fibrosis response. Atrial interstitial fibrosis has been seen in patients with CHF and in animal models of pacing-induced heart failure. It was demonstrated that TGF-beta 1 levels were increased in the atria after the development of congestive heart failure in dogs. In a similar study, mice with increased expression of TGF-beta 1 were prone to atrial fibrillation development as a result of raised levels of atrial fibrosis. If the hypothesis is confirmed, administration of TGF-beta 1 monoclonal antibodies may be used to eliminate the etiology. It will be a new target point to treat atrial fibrillation.