Histiocytoid cardiomyopathy is a rare cardiac disorder of infancy and childhood that predominantly affects girls under the age of 2 years. The clinical picture is usually dominated by severe cardiac arrhythmias, and sudden death may occur. In such instances, sudden infant death syndrome (SIDS) is often considered. Grossly, the affected heart usually shows small, multifocal, subendocardial or myocardial yellow-tan nodular lesions or poorly defined plaques. Occasionally, the lesional tissue is not apparent. Histological findings include sharply demarcated groups and sheets of cells with abundant foamy or granular cytoplasm, justifying the use of the term "histiocytoid" cardiomyopathy. The abnormal cells appear to be transformed cardiac myocytes that possess some features of the conducting system fibers; therefore, Purkinje cells are now believed to be the origin of these distinctive lesions. We describe and discuss the light-microscopic, immunocytochemical, and ultrastructural findings in a previously healthy 12-month-old boy who died suddenly.