The insulin sensitivity of five essential hypertensive patients was compared to five patients with renovascular hypertension, five patients with primary hyperaldosteronism, and five normotensive subjects, using the euglycemic hyperinsulinemic clamp technique. Essential hypertensive patients had significantly lower insulin sensitivity than patients with hyperaldosteronism and renovascular hypertensive patients (P = .0066, P = .004, respectively). Hyperaldosteronism patients also had less insulin sensitivity than renovascular hypertensive patients (P = .016). A significant negative correlation was found between body mass index and insulin sensitivity index for essential hypertension patients only (r = -0.87, P < .003). No such correlation was found in the secondary hypertension patients. The findings suggest a causal relationship between insulin resistance and the development of essential hypertension. Secondary hypertension, on the other hand, is not such an insulin resistant state. Am J Hypertens 1992;5:26-28