Muscular interventricular septum is an extremely rare location for congenital aneurysm. A 48-year-old patient with a muscular interventricular septum aneurysm coexisting with atrial septal defect, pulmonary stenosis and ventricular preexcitation is presented. Three-dimensional echocardiographic reconstruction of the lesion aided in a precise morphologic and quantitative assessment of the condition. The review of 10 previously reported patients with the entity is presented.