Histiocytoid cardiomyopathy (HICMP) is a rare, genetic, cardiac disorder of infancy or childhood, predominantly affecting girls, and clinically manifesting as severe cardiac arrhythmias or dilated cardiomyopathy. Pathoanatomically, HICMP is characterized by subendocardial, epicardial, or valvular yellow-tan nodules, which are histologically built up of abnormal Purkinje fibers and multiple, scattered clusters of histiocytoid myocytes, which are filled with an increased number of normal or abnormal mitochondria. Within the myocardium, yellowish areas with irregular outlines are found and are histologically built up of enlarged, polygonal, histiocyte-like cells with foamy granular cytoplasm. Since HICMP is frequently found in patients with mitochondrial deoxyribonucleic acid (DNA) mutations, HICMP cardiomyocytes carry an increased number of normal or abnormal mitochondria, and may show markedly decreased succinate-cytochrome c reductase or NADH-cytochrome c reductase activity; HICMP should be regarded as mitochondrial cardiomyopathy. Copyright © 2008 Wiley Periodicals, Inc.