Research on atrial histology of humans without cardiovascular disease is scarce. Therefore, our aim was to study human atrial histology in subjects without cardiovascular disease. Histology of the right atrium, left atrium or atrial septum was studied in eight patients (one newborn infant and seven adults) who died of a non-cardiac cause and who were not known to suffer from any cardiovascular pathology. Staining with hematoxylin phloxine saffron or Masson's trichrome was performed to have a better identification of fibrosis and H&E for better identification of lymphocytes. Atrial histology was compared with the histology of the left ventricle and was taken from a collection of standard glass slides. Common light microscopic examination and numeric image processing was performed in all samples. Left atrial histology showed a substantial amount of adipocytes and interstitial fibrosis, associated with replacement fibrosis in some of these cases including one case of lymphocytic infiltrates, similar to the histologic changes of the right ventricle (RV) known in patients with arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVD). Furthermore, we identified a perpendicular orientation of atrial myocardial fibres, which is also a feature of the thin RV free wall. A similar histologic substrate to the RV myocardium known in ARVD is found in the atria of humans without an overt cardiovascular pathology. This may explain the high prevalence of atrial fibrillation in the general population.