Aneurysm-like kialations of the membranous septum are described with increasing frequency in association to a ventricular septal defect (VSD). Their etiology may be congenital, though with different pathogenesis and acquired. An anatomical distinction between true aneurysm of the membranous septum and pseudoaneurysm is proposed. The acquired forms may be joined to a decrease of left to right shunt and are regarded as the mechanism of closure of a VSD. Nine cases of aneurysm-like dilation of the membranous septum are presented: an interpretation of their etiology and pathogenesis is discussed, on the basis of the history and clinical and instrumental data.