In patients with nonvalvular atrial fibrillation, the electrocardiographic fibrillatory wave reflects the structural and electrical remodeling of the atria. This study examined whether the fibrillatory wave amplitude could predict the risk of thromboembolism and demonstrated that this amplitude was related to the duration of atrial fibrillation. We also showed that the presence of fine fibrillatory waves (<1 mm in amplitude) could predict the thromboembolic potential in patients with chronic nonvalvular atrial fibrillation.