Anigographically the sac was like a glove-finger, a cauliflower, a diverticulum, or dome-shaped. Perioperative exploration of 7 cases revaled a perforated true aneurysmal sac. In three patients, perimembranous tissues were severely fibrotic, thickened or having more than one perforations. Two patients possessed a defect under the septal leaflet of the tricuspid valve at the postero-inferior part of the septum, hidden between the chordae of the valve. Defining clearly the membranous septum pathologies is not always possible, by cine-angiography. Echocardiography provides additional information for operative decision. Pathological examination of the resected specimens showed almost total loss of elastic fibers and extensive accumulation of mucopolysaccharides.