Hypertension and diabetes are common diseases in Westernized civilizations, and in the United States, the frequency of both diseases is increasing as the society ages. Factors contributing to the high prevalence and increasing frequency of these diseases include obesity, hyperinsulinemia and insulin resistance, genetic factors, and abnormal cellular handling of calcium and other cations. Obesity is a strong early predictor for the development of hypertension as a person progresses from childhood into adult life. Important factors contributing to obesity-related hypertension likely include enhanced sympathetic nervous system activity and insulin resistance and hyperinsulinemia. Recent evidence has also shown that many nonobese adults with untreated hypertension have insulin resistance and hyperinsulinemia. This observation strongly suggests that the disease called "hypertension" is characterized by fundamental metabolic abnormalities as well as by hemodynamic abnormalities. Recent observations have shown that impaired cellular responses to insulin are associated with increased vascular smooth muscle contraction. Insulin appears to attenuate the vascular response to both receptor-mediated and voltage-mediated calcium-induced contractions. Thus, insulin resistance, and the resultant reduction in the normal attenuating effect of insulin on vascular smooth muscle responses, appear to be associated with abnormal vascular smooth muscle handling of calcium and with exaggerated vascular contraction.