The membranous portion of the interventricular septum represents the final phase of the ventricular growth. It is situated between the orifice of the coronary sinus and the supraventricular crest; immediately below the right aortic semilunar valves and not the coronary. Four cases of aneurism of the membranous portion of the interventricular septum are presented; in two, the diagnosis was made by angiocardiography study and in the rest it was made with the findings of an autopsy. All were of the female sex. Two patients presented a systolic murmur in the low mesocardia; three had heart failure, two of which were secondary to an arteriovenous short circuit through an interventricular communication, and the other due to alternations in the automatism and in the atrioventricular circulation. One case had W-P-W, type A and during its evolution presented paroxysms of atrial fibrillation and flutter, variable degrees of atrioventricular block with Stokes-Adams syndrome and ventricular fibrillation. In one case an obstruction at the level of the outflow tract of the right ventricle was suspected through phonocardiographic studies, and was confirmed subsequently with hemodynamic study. This same case presented a protosystolic aortic snap, at 0.13-0.14 sec. of the q wave of the electrocardiogram, described as of value for the diagnosis of this malformation. In two cases the angiocardiographic study showed the presence of the aneurism in the membranous portion of the interventricular septum, in one, it was visualized in the posterioanterior projection and in another in the lateral. One of the specimens had the aneurism adhered to the tricuspid septal valve, and also a fissure which communicated the left ventricle with the right atrium. In the other, the aneurismal sac was located below the septal valve of the tricuspid, producing a distortion in the anatomical architecture of the atrioventricular orifice.