Extensive tissue oxygenation in the mitochondrial myopathy patients caused by the mitochondrial DNA mutations was first demonstrated noninvasively by a tissue oxymeter measuring near infrared light. The extent of oxygenation of the tissue due to dysfunction of mitochondria correlated with the seriousness of mitochondrial DNA mutations resulting in defects in oxidative phosphorylation system, and causing suppressed oxygen utilization. Such oxygen stress furthers mitochondrial DNA mutations during the progressive course of the disease. This noninvasive diagnosis will find useful application in the diagnosis and management of patients of advanced age.