Lipomatous hypertrophy of the interatrial septum, a finding associated with obesity and advancing age, consists of accumulation of adipose tissue including fetal adipose tissue in the interatrial septum. It is a rare lesion of the heart and can reach notable size. A case is reported in which the diagnosis of lipomatous hypertrophy of the interatrial septum was established intraoperatively; the large bulk of the lipoma was such that it required major reconstruction of the interatrial septum and right and left atrial walls.