Aneurysms of the membranous portion of the interventricular septum are considered to be rare cardiac lesions. Left ventricular angiography appears to be the only method to establish the specific intravital diagnosis. Rupture, infection, thromboembolism, and obstruction of the right ventricular outflow tract have been documented for complications of this malformation. Early resection should be performed before complications occur. 4 patients underwent successful open heart correction of the aneurysm. In another patient a bundle branch block was present and a pacemaker was implanted.