Repeat hemodynamic determinations were performed in 37 young men with borderline hypertension. The duration of the follow up study was 47 +/- three months. At each determination, those with borderline hypertension were compared to a group of matched normal subjects. Blood pressure increased from the first to the second determination, but the increase was significant only for systolic (P less than 0.001) and mean (P less than 0.01) arterial presssures. Cardiac index and heart rate, which were initially increased, decreased significantly (P less than 0.02; P less than 0.01, respectively) and decreased to normal values; total peripheral resistance increased (P less than 0.01); blood and plasma volumes decreased (P less than 0.01). At the first determination, the cardiac index-heart rate correlation was significant (P less than 0.01) and the cardiac index-blood volume correlation was not. At the second determination, on the contrary, the cardiac index-total blood volume correlation was significant (P less than 0.001) whereas the cardiac index-heart rate correlation was not. The study provides evidence that patients with borderline hypertension, over a short-term period, show (1) a greater increase in systolic than in diastolic pressure, (2) a return of cardiac output toward normal values through a decrease both in heart rate and blood volume, and (3) an increased importance of volume factors in the cardiac output control.