Heart failure is important in determining the prognosis of cardiomyopathy caused by mitochondrial gene abnormalities. We report herein the case of a patient with pericardial effusion and heart failure in whom mitochondrial cardiomyopathy was definitively diagnosed. A 56-year-old woman consulted her primary physician with exertional dyspnea. Examination revealed edema and pericardial effusion, and diuretics were prescribed. However, after marked left ventricular hypertrophy (LVH) was noted, she was admitted to our hospital for further evaluation. Further examination revealed short stature, ptosis, generalized muscle atrophy, and sensorineural hearing loss. Echocardiography showed LVH, a global decrease in wall motion, and pericardial effusion. Physical and laboratory findings, including glucose intolerance and elevated serum lactate, suggested mitochondrial cardiomyopathy. Genetic testing confirmed cardiomyopathy due to a mitochondrial a3243g mutation. After treatment to improve heart failure, marked washout was shown on (99m)Tc-MIBI (methoxyisobutylisonitrile) myocardial scintigraphy, suggesting a correlation with mitochondrial dysfunction.