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A 60-year-old woman was admitted for treatment of congestive heart failure. She had been diagnosed with diabetes mellitus when she was 23 years old, and she began to go deaf when she was 34 years old. She showed symptoms of heart failure at age 51 and was diagnosed with hypertrophic cardiomyopathy. Echocardiography showed progressive diffuse hypokinetic motion of the left ventricle and the left ventricular hypertrophy had gradually regressed. A mitochondrial transition mutation, A3243G, was detected in her peripheral leukocytes (9%) and in those of her 27-year-old son, who also has diabetes and deafness. Electron microscopy of an endomyocardial biopsy specimen showed proliferation and swelling of the mitochondria, and significant generation of reactive oxygen species (ROS), as well as marked induction of heme oxygenase-1, which is an adaptive enzyme to oxidative damage, were also observed in the myocardial tissue. These observations were more prominent than in other patients with heart failure of different etiology, which suggests that the increased ROS generation and anti-oxidative response were involved in the development of the mitochondrial cardiomyopathy.