This double-blind randomized trial was carried out on 37 patients with diabetic retinopathy and blood hyperviscosity. 19 patients received calcium dobesilate in capsules of 500 mg 3 times daily for 3 months and 18 patients took 3 placebo capsules daily during the same period. Calcium dobesilate was found to reduce whole-blood viscosity in a statistically significant manner which was accompanied by a diminution of the albumin/globulin ratio and a significant reduction of fibrinogen and cholesterol levels. A statistically significant diminution of capillary fragility was also observed in the calcium dobesilate-treated group. These results indicate that calcium dobesilate, by restoring the integrity of the microvessels, and by lowering blood viscosity, could act favorably on the evolution of diabetic retinopathy.