There is direct evidence that there is an increase of concentration of oxidizing species and oxidized products in plasma of human diabetics. The extent of this increase seems to reflect a predilection to diabetic damage. 1. A high concentration of lipid hydroperoxide in plasma was observed in diabetic patients and it's levels correlated well with the degree of diabetic nephropathy. 2. Lipid peroxide causes membrane injury of endothelial cells. The addition of anti-oxidant inhibited cell injury markedly. 3. Malondialdehyde and protein (lysin-residual or low density lipoprotein) made conjugates to change the antigenicity. This results shows the possibility that atherosclerosis as diabetic complication may be caused by immunological reactions with modified proteins for example, oxidized LDL and so on. 4. SOD activity in erythrocytes of diabetic patient was extremely decreased compared with non diabetics, but no difference was observed by the ELISA method with monoclonal antibody. Glycosylation had been expected to occur in various kinds of proteins. The inactivation of SOD may be caused by non enzymatic glycosylation, because negative correlation was observed between the activity of SOD and GHb in erythrocytes. This inactivation of SOD may play an important role in the pathogenesis of diabetic complications. From these results, it was suggested that both free radical reactions and non enzymatic glycosylation may play important roles not only in the development of diabetes but also in its complications.