Lipomatous hypertrophy of the interatrial septum ("massive fatty deposits in the atrial septum") may represent an incidental autopsy finding or may be associated with atrial arrhythmias, obstructive symptoms, or sudden death. Antemortem diagnosis is becoming much more common. In a literature review and in a review of our 12 cases, this condition was found to occur more frequently in obese middle-aged or elderly subjects, who frequently demonstrated atherosclerotic coronary artery disease. The unencapsulated mass generally occurred anterior or superior to the fossa ovalis, and histologically was characterized by mature fat with varying quantities of fetal fat, inflammation and fibrosis, and entrapment of myocardial fibers with cytologic atypia. Cases were included that had a minimum atrial septal thickness of 1.0 cm. The septum ranged up to 7.0 cm in maximum thickness (2.6 cm average). In our cases, no deaths were directly attributed to this disorder. It is important that forensic pathologists become more generally aware of this condition. To further delineate the true pathological significance, the atrial septum should be routinely examined by palpation and sectioning to determine its thickness and fat content.