Diabetic patients grouped according to the evolutive stages of their retinal involvement were examined by a direct method for estimation of segmental retinal blood flow by slit-lamp fluorophotometry. Initial stages of retinal involvement in diabetes, in which the only alteration is a breakdown of the blood-retinal barrier, show values of segmental blood flow that are within normal limits. Patients with a minimal background retinopathy show a slight increase in retinal blood flow, whereas an highly significant increase of segmental blood flow is observed in the more advanced stages of background retinopathy with maculopathy. In general, a good correlation is apparent between the progressions of diabetic background retinopathy and increase in retinal blood flow. On the other hand, patients with proliferative retinopathy show lower values of retinal blood flow, which are attributable to a marked decrease in arterial diameter.