The complexity of the pathogenesis of diabetic retinopathy (DR) makes it difficult to produce effective drugs for its treatment. Among the active principles recently suggested, the peptide fraction of bovine factor VIII of the clotting cascade deserves particular attention. Bovine factor VIII derivatives (vascular factor, VF) have been shown to have significant effects on the capillary basement membrane and on vascular endothelium and they are able to stabilise vessel walls. The aim of this study was to assess the effectiveness of VF in diabetic patients with non-proliferative DR. Nineteen consecutive patients (37 eyes) affected by non-proliferative DR were included in the study and randomly assigned to VF or placebo. An ophthalmological examination, colour photographies and a retinal fluorescein angiography were carried out before as well as 3 and 6 months after inclusion. Every eye in the study was classified in accordance with the ETDRS criteria. While the progress of macular oedema and visual acuity was very similar in the two groups (VF and placebo), there was no deterioration, i.e., no change to a worse classification than that at the start, in any of the eyes in the group treated with VF, but deterioration was seen in 19% of the eyes treated with the placebo. On the basis of these results, VF may ensure some protection of the blood-retinal barrier against the harmful influence of diabetic microangiopathy.