Concerning the pathogenesis of microangiopathy, it is possible to assume that the metabolic failure due to an insufficient insulin action appears first and then vascular lesion follows, to this state. The relationship between an insufficient insulin action and morphological and chemical changes in capillaries, paticularly in glomerulus, retina, muscle and skin, have been pointed out and studied as the causes of microangiopathy. Of late, however, attention has been paid to microcirculatory failure as a possible cause; abnormal viscous flow, intravascular red cell aggregation, hypercoagulability and low fibrinolytic activity of blood are being regarded as important factors in the development of microcirculatory failure and investigation from the novel viewpoint of hemorheology has been made. This paper concerns some studies that attempt to correlate blood sludging in eye bulbar conjunctiva with the extent of the hemorheological abnormalities and the severity of retinopathy in patients with diabetes mellitus. The degree of sludging in bulbar conjunctiva and its correlation with blood and plasma viscosity, platelet adhesion and aggregation, blood clotting curve, plasma fibrinogen concentration and erythrocyte sedimentation rate were investigated in 16 normal healthy subjects and 48 diabetics. For the observation of blood sludge in conjunctiva vessels, our newly-developed biomicroscope (magnification ×50) was used, and then classified arbitarily into four categories according to the extent of intravascular red cell aggregation in arterioles, capillaries and venules.The patients with diabetes were divided into two groups: DM-I, a group without retinopathy and/or with mild retinopathy, consisted with O-II stages in Scott's classification of diabetic retinopathy, and DM-II, with advanced retinopathy (Scott III-V). Diabetes groups showed significantly increasing of the parameters such as blood and plasma viscosity, platelet aggregation, maximum dynamic rigidity modulus of blood clotting and fibrinogen concentration than those of normal subjects. However, no significant differences of these parameters were found between DM-I and DM-II groups, though latter having a tendency for more abnormalities in blood rheology. Significant sludging in conjunctival vessels were observed in most of DM-II group and half DM-I group: It was assumed that there were some correlation between blood sludge and the severity of diabetic retinopathy. And in the relation of clinical factors high incidence of sludging grade 2 or higher was observed in diabetics with long duration, albuminuria and high triglyceride. Other factors such as blood sugar level in fasting, body weight and total cholesterol have no significant correlation with degree of sludging.