Thirty-eight cases of lipomatous hypertrophy of the cardiac interatrial septum are presented and analyzed, together with those previously reported. Available data indicate that this entity is more likely to be seen in the seventh to eighth decade of life, in Caucasians, and with age-associated increases of epicardial fat. It differs from cardiac lipoma, which is a true neoplasm occurring in a younger age group. It is probably more frequent than reported as it is usually not looked for. Atrial arrhythmias sometimetimes coexist with this lesion, but a cause-and-effect relationship would be difficult to prove. The lesion should be included in the differential consideration of atrial masses found by ultrasound, cineangiocardiogram, and gated radionuclide cardiac imaging.