ACE (angiotensin converting enzyme) gene genotypes have been shown to be a risk factor for development of left ventricular hypertrophy and cardiomyopathy, upon the assumption that the DD genotype is linked to higher cellular ACE activity leading to myocardial fibrosis. To test an analogous hypothesis that the DD genotype favors myocardial fibrosis in the atrium and thus promotes atrial fibrillation without any structural heart diseases, we determined the distribution of the ACE gene genotypes in 77 patients with lone atrial fibrillation and investigated the effects of the ACE genotypes on the clinical characteristics of atrial fibrillation. The distribution of ACE genotypes was not significantly different between the patients and healthy volunteers. Also, the ACE gene genotypes did not affect the types of atrial fibrillation and the age at the onset of atrial fibrillation. Thus, these results refuted the hypothesis of possible relationships between ACE genotypes and lone atrial fibrillation through myocardial fibrosis, and indicated some unknown differences between the atrium and ventricle.