The sudden death of an infant may be due to a variety of causes. In such cases, complete autopsy with radiologic, gross, microscopic, and toxicologic examination is warranted. We present a case of a previously healthy 15-month-old girl with no known disease, who experienced witnessed cardiac arrest, ventricular fibrillation, and death. Complete postmortem examination revealed histiocytoid cardiomyopathy as the cause of death. Histiocytoid cardiomyopathy is a rare infantile cardiac-muscle disorder characterized by the presence of enlarged, polygonal subendocardial myocytes which, by light microscopy, lack normal striations, and instead have granular, faintly eosinophilic cytoplasm. Ultrastructurally, the myocytes contain numerous mitochondria and markedly reduced numbers of myofibrils. Clinically, the disorder is characterized by cardiac arrhythmias and/or sudden death occurring in children under the age of two years. We discuss the differential diagnosis, proposed theories of etiology, and the pathology of this rare entity.