Hyperinsulinemia or insulin resistance is suggested to play a role in the pathogenesis of hypertension and its target organ diseases. We designed this study to evaluate the role of insulin resistance in cardiac function, cardiac hypertrophy, wall thickness of the common carotid artery, and endothelial function of the brachial artery in essential hypertensive patients without diabetes mellitus. Insulin resistance was evaluated by the constant glucose infusion rate (M value) during the euglycemic-hyperinsulinemic glucose clamp test. In correlation analysis for several indices of glucose metabolism, only M value correlated with left ventricular mass index (LVMI), ratio of peak velocity during atrial contraction to that during early left ventricular filling phase (E/A ratio) and intima-media complex (IMC). In stepwise regression analysis of various risk factors for cardiovascular diseases, only M value and age were dependent factors for LVMI, E/A ratio, and IMC. No indices of glucose metabolism or risk factors for cardiovascular diseases correlated with endothelium-dependent and -independent vasodilation. These results suggest that insulin resistance, but not glucose intolerance and hyperinsulinemia, partly accelerates cardiovascular complications such as left ventricular hypertrophy and wall thickening of the carotid artery in patients with essential hypertension.