Mitochondrial diseases may cause hypertrophic cardiomyopathy or, less commonly, dilated cardiomyopathy. The authors describe a case of a 46 year old woman with dilated cardiomyopathy associated with diabetes and deafness. This association is suggestive of the MIDD syndrome and the demonstration of metabolic abnormalities (raised serum lactic acid, abnormal lactate/pyruvate ratio) was compatible with a mitochondrial disease. This diagnosis was confirmed by finding an A3243G mutation of the mitochondrial DNA.