Obesity and hypertension are closely associated. Hypertension occurs frequently in industrialized populations that gain weight with advancing age, and is infrequent in primitive populations that are not obese. There are two reasons for concern about the relationship of obesity to hypertension. Weight gain in young adult life is a potent risk factor for later development of hypertension. Weight reduction in obese hypertensive persons often reduces arterial pressure. Mechanisms of obesity hypertension are as yet unidentified; an earlier hypothesis that it is related to salt intake has not been supported by recent studies. Hemodynamic studies have shown that obesity is associated with an elevated cardiac output and expanded blood volume; in normotensive obese persons peripheral vascular resistance is reduced, and in hypertensive persons it is normal or elevated. Studies of hormonal and neural factors have failed to explain the presence of hypertension in some obese persons and its absence in others.