The case of a 24 years-old -woman with weakness since the teens and progressive loss of muscle strenght is reported. The muscle biopsy showed increased number of mitochondria. In two occasions the respiratory chain enzymes showed important reduction of the succinate-cytochrome-C-reductase, suggesting a possible defect in the complex II of the respiratory chain. Large doses of vitamins C and K were prescribed. There was improvement of muscle strenght. A discussion about the most common syndromes marked by mitochondrial abnormalities in muscle is made, as well as about the type of work-up that should be done in suspect cases of respiratory chain defects.