A four-month-old female with no previous medical history suddenly collapsed and failed to recover despite 2h of resuscitation. An autopsy showed marked cardiomegaly (88g) with prominent dilatation of the left ventricle and a whitish opacity on the endocardial surface. The ductus arteriosus was patent, but both orifices were severely stenosed. Microscopically, the endocardium showed pronounced thickening with laminar deposition of elastic and collagen fibers. Additionally, there was a mixture of myocardial fibers with a marked "wavy" appearance and a scattering of mild interstitial lymphocytic infiltration. We believe that endocardial thickening in this infant met the diagnostic criteria for endocardial fibroelastosis (EFE). Although it is controversial whether primary EFE is a distinct pathologic entity or an epiphenomenon, we speculated that "dilated cardiomyopathy with EFE" had caused the decedent's death based on the appearance of the myocardial fibers.