Aldose reductase (AR) has been implicated in the etiology of diabetic cataract, as well as in other complications. However, the role of AR in these complications remains controversial because the strongest supporting evidence is drawn from the use of AR inhibitors for which specificity in vivo cannot be ascertained. To settle this issue we developed transgenic mice that overexpress AR in their lens epithelial cells and found that they become susceptible to the development of diabetic and galactose cataracts. When the sorbitol dehydrogenase-deficient mutation is also present in these transgenic mice, greater accumulation of sorbitol and further acceleration of diabetic cataract develop. These genetic studies demonstrated convincingly that accumulation of polyols from the reduction of hexose by AR leads to the formation of sugar cataracts.