OBJECTIVE To investigate the correlation between adult diabetic cataracts and levels of aldose reductase (AR) in red blood cells (RBCs). METHODS The study involved 337 eyes of 337 patients with diabetes. The extent and severity of lens opacity was assessed according to the Lens Opacities Classification System III (LOCS III). The AR levels within RBCs were determined with an ELISA. The relationship between the AR level in RBCs and the prevalence of nuclear cataract, cortical cataract, and posterior subcapsular cataract in patients with diabetes was examined. RESULTS There were no significant alterations in AR level in RBCs in patients with a diabetes duration of < or = 10 years and patients < 60 years of age. In each subgroup, a higher amount of AR levels in RBCs significantly correlated with the prevalence of posterior subcapsular cataracts. A significant association between cortical cataract and AR level in RBCs was also seen in a subgroup of patients younger than 60 years. CONCLUSIONS AR emerges as an important factor affecting the onset of posterior subcapsular cataracts at the early stages of diabetes mellitus. This raises the possibility that AR inhibitors could play a useful role in treatment of adult diabetic cataract through its inhibition of AR activities.