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Mitochondrial cardiomyopathy (MCM) is usually recognized as one of the phenotypes of systemic mitochondrial disease. However if there are no cardiac symptoms, it is difficult to make a definite diagnosis because of various cardiovascular phenotypes and no diagnostic criteria in pathological examination. To add myocardial pathology to the diagnostic criteria for mitochondrial respiratory chain disorders, which is the gold standard in the diagnosis of mitochondrial diseases, we performed quantitative analysis of mitochondria using electron microscopy and immunohistopathologic analysis with respiratory chain enzyme antibodies. Ten patients with hypertrophic or restrictive cardiomyopathy who had undergone endomyocardial biopsy were studied. Respiratory chain enzymatic assay and genetic study were performed and four patients were diagnosed with MCM. Using electron microscopy with quantitative analysis, volume density of mitochondria within cardiac muscle cells was significantly increased in the MCM group compared to the non-MCM group (p=0.013). Immunohistopathologic results were compatible with the result of the respiratory chain enzymatic assay. These advanced pathological tests can distinguish MCM from other cardiomyopathies. Results of immunopathologic study Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): The Practical Research Project for Rare/Intractable Diseases from the Japan Agency for Medical Research and Development, AMED.