The viscosity of the whole blood, plasma and serum, haematocrit and plasma fibrinogen were studied in diabetic patients with (DR) and without (D) retinopathy and in non-diabetic control subjects (C). Blood viscosity was significantly higher in diabetics than in controls. No significant differences in viscosity of the whole blood were found when various types of retinopathy were compared according to the severity of retinal damage. Plasma viscosity was significantly higher (P less than 0.01) than controls (C) only in diabetic patients with retinopathy (DR). Serum viscosity was significantly increased compared with controls (C) only in diabetic patients affected by proliferative retinopathy. Plasma fibrinogen was significantly higher than controls (C) both in diabetics with retinopathy (DR) and without retinopathy (D). Haematocrit did not show a significant difference in the three groups considered (C, D, DR).