A closed interatrial septum aneurysm, filled with blood, mimicking a tumour in the right atrium.

We report the case of a 70-year-old woman with rheumatic mitral stenosis and a transient ischaemic attack. Transoesophageal echocardiography revealed a cystic mass in the right atrium, hanging to the interatrial septum by a pedicle, not circulating. The mass was heterogeneous and suggested a tumour (myxoma) or a thrombus. Surgical resection showed it was an interatrial septal aneurysm, closed on itself, filled with blood. The usual causes of cardiac tumours and pathogeny of large interatrial aneurysms are discussed.