In order to identify the hemodynamics of borderline essential hypertension, radionuclide angiography was performed before and after bolus injection of furosemide (40 mg i.v.) both at 3 min (i.e. before diuretic effect) and at 30 min (i.e. after diuretic effect) in 16 borderline (B) patients and in 14 age-matched WHO classification I-II essential hypertensives (H) patients. 14 age-matched normotensive (N) subjects were used as controls. B patients were further subdivided into two subgroups according to a cardiac index under or above 3 liter/min/m2 in basal conditions. Baseline hemodynamic characteristics showed higher values of mean arterial pressure (MAP) and systemic vascular resistance index (SVRI) in both H and B patients when compared with N subjects (p < 0.001). Furthermore, B and H patients exhibited lower values of left ventricular peak filling rate (PFR) than seen in N subjects (p < 0.01 and p < 0.05, respectively). H patients demonstrated higher peak systolic blood pressure/end-systolic volume ratio (PSP/ESV) than seen in N subjects (p < 0.05). PFR positively correlated with peak emptying rate (PER) only in N and B patients (p < 0.05). After furosemide administration, even though differences were observed in the absolute values, B and H patients showed similar hemodynamic patterns. Only the B subgroup with cardiac index (CI) > 3 liter (‘volume-dependent’ patients) showed a decrease in left ventricular end-diastolic volume index (LVEDVI) at 30 min associated with a lowering of stroke index (SI; p < 0.005 for both), when compared with pre-drug values. In B patients with CI < 3 liter (‘afterload-dependent’ patients) no differences were observed either at 3 min or at 30 min in comparison with values obtained prior to drug administration. Moreover, in this subgroup, like in H patients, there was a negative correlation (p < 0.01) between 3-min percent change of SVRI and 3-min percent change of SI. Our data suggest that in ‘borderline’ hypertension: (a) there may be an increase in peripheral resistance, as in established hypertension, especially when age-matched groups are considered; (b) the earliest sign of compromised left ventricular function is the reduction in dia-stolic PFR but, unlike established hypertension, this index is still correlated with systolic function; (c) cardiac output might be even somewhat reduced and also negatively correlated with vascular resistance (‘afterload-dependent’ hearts); (d) furosemide (acute administration) might contribute to a better definition of hemodynamic behavior.