Patients with borderline hypertension have decreased plasma volume. This is a reproducible finding. Decreased plasma volume is associated with increases of the hematocrit reading. Increased cardiac output and heart rates were observed in borderline hypertension, but this was not associated with the decrease of the plasma volume. However, the decreased plasma volume was correlated with increases of the blood pressure and of the total peripheral resistance. Since the decrease of plasma volume clearly relates to indices of "severity" of borderline hypertension (blood pressure and resistance), measurement of plasma volume may be important for understanding the natural history of the disease and for the assessment of an individual's risks.