During the last two decades, vision researchers have studied intensively the role of the enzyme aldose reductase (AR) in diabetic cataract and in other diabetic complications. The potentially damaging effects of AR in diabetes were first discovered in the lens fiber cells. In these cells, the enzyme catalyzes the reduction of high levels of glucose to sorbitol through the polyol pathway and initiates a sequence of events that eventually results in opacification of the lens. Because sorbitol (a sugar alcohol) and fructose (a subsequent product of glucose metabolism) cannot readily diffuse across cell membranes, they become trapped in lens fiber cells. There they accumulate at unusually high levels and exert a significant osmotic stress, causing an influx of water and swelling of the lens fibers. Major electrolytic and osmotic changes then occur that lead to lens opacity.