A 65-year-old man was scheduled for an on-pump coronary artery bypass graft procedure because of a three-vessels coronary artery disease. A right atrial mass appended to the interatrial septum was discovered during intraoperative transoesophageal echocardiography. Therefore, the right atrium was opened. Gross examination revealed a fatty lesion of the interatrial septum. A biopsy was performed before the atrium was closed. A histological diagnosis of lipomatous hypertrophy of the interatrial septum was made. Lipomatous hypertrophy of the interatrial septum is a mass of adipocytes infiltrating the interatrial septum. The aspect of "dumbbell" produced by the sparing of the Fossa Ovalis is typical. The lesion is benign and remains asymptomatic most of the time although it can be responsible for cardiac arrhythmias or circulatory obstruction. The typical echographic aspect should be known to avoid unnecessary surgical resection.