INTRODUCTION: Early repolarization syndrome is a recently proposed condition characterized by an early repolarization pattern in the electrocardiogram (ECG) and ventricular fibrillation in the absence of structural heart abnormalities. Although some studies have suggested that early repolarization is associated with frequency of atrial fibrillation, the association of early repolarization with atrial fibrillation is not well known. HYPOTHESIS: Early repolarization indicates the substrate for atrial fibrillation in addition to that for ventricular fibrillation. METHOD: This study included 79 patients (57 men [72%]; age, 45 ± 12 years) aged less than 60 years who had paroxysmal lone atrial fibrillation and 395 age- and sex-matched healthy controls (patient:control ratio, 1:5). Patients who had structural heart disease, hypertension, diabetes, hyperthyroidism, history of successful resuscitation, or the Brugada type ECG were excluded. ECGs recorded during sinus rhythm were compared between patients with atrial fibrillation and healthy controls. RESULTS: Early repolarization in the inferior and/or lateral leads was more common in patients with atrial fibrillation (25%) than controls (10%; P = 0.001). The location and magnitude of early repolarization were similar between the two groups. Other electrocardiographic measurements were not different between the two groups. Among patients with atrial fibrillation, there was no difference in clinical characteristics including age at atrial fibrillation development, sex, and body mass index between patients with early repolarization and those without early repolarization. Electrocardiographic measurements were not different between patients with early repolarization and those without early repolarization. CONCLUSION: Early repolarization was associated with lone atrial fibrillation. Early repolarization may indicate increased susceptibility to atrial fibrillation.