We examined plasma levels of fibrinopeptide A, beta-thromboglobulin, and platelet factor 4 in diabetic patients. Among diabetic patients (n= 33) plasma levels of fibrinopeptide A, beta-thromboglobulin and platelet factor 4 were significantly higher than in controls (n= 41). In the subgroups of diabetic patients with (1) minimal (n= 13), and (2) moderate-severe (n= 14) retinopathy only plasma fibrinopeptide A levels were significantly higher than in controls. Among the total group of diabetic patients plasma levels of fibrinopeptide A increased significantly with increasing severity of retinopathy. These results suggest that diabetic retinopathy is associated with in vivo activation of blood coagulation factors and that this activation increases with advancing retinopathy.