It is now well recognized that insulin resistance and/or hyperinsulinemia are characteristic of a number of common human disease states including obesity, non-insulin dependent diabetes mellitus (NIDDM), essential hypertension, and atherosclerotic cardiovascular disease. More recent evidence suggests that impaired insulin action and elevated levels of circulating insulin may also be present in a substantial proportion of apparently healthy nonobese individuals. Considerable attention is now being focused on the potential long term adverse consequences of elevated circulating insulin levels. In particular, the frequent concurrence of these clinical disorders of carbohydrate metabolism, lipid metabolism, and vascular disease has led to the hypothesis that insulin resistance and the ensuing hyperinsulinemia may be a common pathophysiologic factor in the etiology of these disease states. In this review, we will examine the evidence for this hypothesis with particular attention to the adverse effects of chronic hyperinsulinemia.