The concentration of copper ion in cataractous lenses was higher than that in clear lenses. The concentration of copper ion was significantly higher in subjects with diabetes (DM group) than in subjects without diabetes (control group). The concentration of Cu2+ ion was significantly greater than that of Cu+ ion in the DM group. Furthermore, the concentration of copper ion unconjugated to protein was higher than that of protein-conjugated copper ion. It is assumed that in the cataractous lenses of the DM group, a decrease in the reactivity of the copper-containing enzyme superoxide dismutase and an increase in hydrogen peroxide concentration leads to the generation of hydroxyl radicals from the Fenton's-type reaction. In diabetes, an increase in lenticular glucose induces glycation with the release of copper ion from copper-containing enzymes, thus increasing the concentration of lenticular copper ion. As a result, superoxide scavenging activity is reduced and peroxide lipid concentration is increased. This is assumed to result from the hyperactivity of the peroxidation cascade.