Systemic hemodynamics, intravascular volume, and plasma renin activity were determined in 135 lean, midly obese, or distinctly overweight subjects who were normotensive or had borderline or established essential hypertension. Cardiac output (but not index) was higher and peripheral resistance lower in obese than in lean subjects, except in borderline hypertension. Intravascular volume was increased in obese patients, and more so when corrected for body height; correction for body weight led to relative volume contraction. Intravascular volume correlated directly with cardiac output in the entire population, as well as in the subgroups. Intravascular volume correlated inversely with total peripheral resistance in all subjects and in each subgroup. Both correlations remained significant when an approximation was used to correct influences of obesity on total blood volume. Sodium excretion was higher in obese than in lean subjects. Thus, despite the expanded intravascular volume in obesity, the pathophysiologic relationship between systemic hemodynamics and intravascular volumes remains unchanged. Relatively low peripheral resistance in obesity may decrease the risk of systemic vascular disease. Nevertheless, since circulating volume is increased, the greater venous return adds an additional load to a left ventricle that is already burdened by a high afterload caused by arterial hypertension.