The vascular chronic complications are the main cause of morbidity and mortality in patients with diabetes mellitus. Nowadays it is well known the fact that the arteriosclerosis is initiated by the injury of the vascular endothelium and that the normal endothelial cells are producing a number of vasoactive factors. Thus, the vascular endothelium was considered an inert "lining" layer, but today is seen as a complex organ, with paracrin and autocrin function, which provides a "first line" physiological defence against atherosclerosis. Impaired endothelial function occurs in people with diabetes as a result of associated conditions (e.g. hyperglycemia, hypertension, dyslipidemia), or as an effect of hyperglycemia itself (e.g. cytokines, free fatty acids, AGES). The presence of endothelial dysfunction in non diabetic insulin resistant subjects suggests that metabolic and vascular abnormalities are tightly related at a fundamental level. Recent evidence suggests that insulin signalling for glucose transport in classical target tissues (muscle and adipose tissue) and upregulation of NO production in the endothelium utilises the same postreceptor pathway. This pathway involves the enzyme P13 kinase. Knowledge of the molecular mechanisms involved in the pathogenesis of endothelial dysfunction may ultimately result in novel approaches to the treatment and prevention of cardiovascular disease in people with diabetes mellitus.