Congenital aneurysms of the muscular interventricular septum are rare. We report the diagnosis of such an aneurysm by fetal echocardiography at 19 weeks' gestation and the subsequent follow-up through early neonatal life, with the ultimate progression to a dilated cardiomyopathy requiring cardiac transplantation. This case highlights the early diagnosis of this rare finding and the unusual clinical course to include the confounding features of endocardial fibroelastosis, left ventricular noncompaction, and a lymphocytic infiltrate demonstrated on pathologic examination. Most infants with congenital septal aneurysms are asymptomatic, but this case demonstrates the need for careful serial assessment and clinical follow-up because of the unclear origin and natural history of this entity.