To study the effects of the new aldose reductase inhibitor tolrestat on diabetic retinopathy, 31 diabetic patients with various degrees of retinopathy were randomly assigned to either tolrestat (200 mg once daily) or placebo treatment for six months. Separate morphological features of diabetic retinopathy were assessed by fundus photography and fluorescein angiography before and at the end of the study. The results showed some amelioration of clinical signs of diabetic retinopathy during aldose reductase treatment. Hard exudates, intraretinal hemorrhages and focal fluorescein leakage increased on average in the placebo and decreased in the tolrestat group. The difference was statistically significant for focal fluorescein leakage only. The permeability of the blood retinal barrier was determined by vitreous fluorophotometry before and at the end of the study. No change in permeability values was found.