Studies were conducted to evaluate the role of water-sodium balance and renal dopaminergic activity in the hypertensive mechanisms of overweight patients with essential hypertension (EHT). The body mass index (BMI) was correlated positively with arterial pressure, plasma volume, extracellular fluid volume, or total exchangeable sodium and negatively with plasma noradrenaline concentration or plasma renin activity in patients with EHT. Fractional excretion of sodium (FENa) was significantly lower in overweight patients with EHT than that in normal-weight patients with EHT. Hypotensive effects of sodium restriction or the natriuretic response to infused dopamine was more remarkable in overweight than in normal-weight patients with EHT. Urinary excretion of free dopamine (UDA) was correlated positively with simultaneously measured urinary excretion of sodium or FENa and negatively with the natriuretic response to dopamine infusion. In addition, UDA was positively correlated with the BMI in normal-weight patients with EHT, whereas the relation between UDA and BMI was significantly negative in overweight patients with EHT. These findings suggest that the expansion of body fluid volume and sodium might result from the blunted natriuretic ability due to an attenuation of the renal dopaminergic activity in overweight patients with EHT. The expansion of body fluid volume and sodium may play an important role in the hypertensive mechanisms of overweight patients with EHT.