In four patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) in which mutated mitochondrial deoxyribonucleic acid was seen, hypertrophic cardiomyopathy and angiopathy was demonstrated by echocardiography, dipyridamole stress scintigraphy, and cardiac catheterization. On stress scintigraphy with dipyridamole, three patients showed hypoperfusion in the early image and a "filling-in" pattern in the late image. However, coronary angiography did not demonstrate narrowing of the large vessels in these patients. Light and electron microscopy of endomyocardial biopsy specimens indicated abnormal mitochondria, with marked increase in the number and size of mitochondria in endothelium. Modified Gomori's trichrome staining in biopsied endomyocardial specimens revealed a red-purple deposit similar in appearance of the ragged-red fibers in skeletal muscle, a characteristic finding of mitochondrial disease. Deterioration of complex I in the mitochondrial electron transfer system, which is widely observed in various mitochondrial diseases, appeared in biopsied skeletal muscle of our patients, indicating deficiency of some subunits of complex I. These results indicate that mitochondrial diseases such as MELAS show not only cardiomyopathy but also angiopathy. We speculate that proliferation of mitochondria leads to narrowing of the lumen of arterioles, which might be responsible for the ischemic findings observed scintigraphically.