ABSTRACT.Altogether, histologic findings in animals and in man with diabetes mellitus, together with clinical findings in man with this disease and with biochemical abnormalities, suggest that the main cause of microangiopathy in persons with diabetes mellitus lies in metabolic disorders. The possibility that genetic and environmental factors are influential cannot, however, be ruled out. In a small although steadily increasing number of diabetics early microangiopathic and neuropathic abnormalities have proved to be reversible. This improvement includes functional abnormalities of the retina, glomeruli and nerves. Early morphologic changes of the retina also, seem to be preventable. On the other hand, more severe morphologic' changes such as proliferative retinopathy, and severe diabetic neuropathy have proved resistant to intensified insulin treatment. It remains to establish whether continuous optimal control by insulin treatment generally can prevent early although clinical microangiopathy. The development of instruments and other aids for improved metabolic control, such as transplantation of insulin‐producing tissue, would seem to be a logical step in this directio