We report 2 brothers with hypertrophic cardiomyopathy, lipid storage and skeletal mitochondrial myopathy. Biochemical analyses detected complex II (succinate dehydrogenase) deficiency of the respiratory chain. Single fiber analyses using a microphotometer were performed and revealed reduced succinate dehydrogenase activity in all muscle fibers. This report shows the validity of kinetic microphotometric analysis in revealing complex II defects.