Interventricular septum aneurysm is a rare, non-syndromic, congenital heart malformation characterized by the presence of a congenital aneurysm of the membranous portion of the interventricular septum. Patients may be asymptomatic or may present with ventricular or supraventricular tachycardia, fatigue, exertional dyspnea, palpitations, and cardiac murmur. Ventricular septal defects and conduction defects, such as first-degree atrio-ventricular block or incomplete right bundle branch block, may also be also associated. The disease is interventricular septum aneurysm (Monarch Disease Ontology entry MONDO_0007112).