There is a close association of obesity with hypertension through mechanisms not now understood. Hypertension occurs frequently in industrialized societies where weight gain with advancing years is a common feature and is rare in primitive societies where weight and age are negatively correlated. Hemodynamically, obesity is characterized by an expanded blood volume and increased cardiac output. Hypertension results if/when systemic vascular resistance fails to decrease as cardiac output increases. When calorie restriction leads to weight loss, both blood volume and cardiac output decrease; when the blood pressure falls, this is because peripheral resistance is unchanged. Weight-loss programs are helpful for the obese hypertensive because when hypertension is mild, blood pressure often (but not always) normalizes. Also, weight loss has been shown to decrease antihypertensive drug requirements. Interest in obesity-associated hypertension focuses on hyperinsulinemia/insulin resistance as causative factors. Although the evidence is tempting, it is far from conclusive and it seems likely that the mechanism of this type of hypertension is as multifactorial as those of other types.