Serum aldose reductase activity was estimated spectrophotometrically in 25 normal subjects (Group A), 25 diabetic patients without retinopathy (Group B), and 25 diabetic patients with retinopathy (Group C). Serum aldose reductase levels were higher in diabetic patients with or without retinopathy than in normal subjects. Comparing the serum aldose reductase activity in diabetic patients with and without retinopathy showed the value was higher in the former group; this difference was statistically significant (P < .01). However, aldose reductase activity in proliferative and background retinopathy were equal and therefore not significant.