The width (Wr/Wo) and the intensity (Ir) of the central light reflex were studied on retinal arteries and veins in insulin-dependent diabetic patients. A diabetic group (n = 45) was compared to normal subjects (n = 57), and the groups were matched according to age and blood pressure levels. A computerized scanning microdensitometer was applied across various sites on vessels in fundus photographs. The Ir of the reflected light was significantly reduced in diabetic arteries and veins compared to the vessels in the control group (P less than 0.001). This may implicate abnormalities in blood rheology of the retinal circulation in diabetes. The Wr/Wo was found narrower in veins in diabetic patients (P less than 0.001). This probably indicates differences in haemodynamics of venous flow in diabetic and normal subjects. The intensity was correlated to the duration of diabetes (P less than 0.05), while neither the width nor the intensity were found related to sex, metabolic control, intraocular pressure or retinopathy.