Twelve adults with mitochondrial myopathies were studied by phosphorus magnetic resonance spectroscopy of muscle. All 12 had abnormal 31P-NMR findings; recovery from exercise was abnormal in 11 patients. At rest, the ratio of phosphocreatine to inorganic phosphate was reduced in 10. Exercise transfer characteristics were abnormal in all five patients who could exercise. Exercise-induced intracellular acidosis was subnormal in nine patients. The range of abnormalities indicates biochemical heterogeneity, with two possible groups: primary defects of energy metabolism with marked 31P-NMR abnormalities, and secondary, less specific 31P-NMR abnormalities.