The level of thiobarbituric acid-reactive products and the specific activities of catalase and glutathione reductase were significantly higher in muscles from patients with major forms of muscular dystrophies over those of control subjects. Superoxide dismutase activity was not altered in dystrophic muscles. These findings indicate the occurrence in human dystrophic muscles of an increased level of lipid peroxidation and the possible activation of certain enzymes that could conceivably inhibit lipid peroxidation in vivo.