OBJECTIVE It has been suggested that the blood pressure elevation in borderline hypertension is caused by hyperreactivity to stress. We addressed the questions: are subjects with borderline hypertension hyperreactive to mental stress, and, if so, is this reflected in greater blood pressure responses during daily-life activities, and does non-specific pressor amplification by structural vascular changes contribute to reactivity changes? METHODS Standardized mental stress was performed during invasive monitoring in 54 borderline hypertensive subjects [systolic blood pressure (SBP) 140-160 or diastolic blood pressure (DBP) 84-95 mmHg, or both] and 20 normotensive control subjects (110-130/60-80 mmHg). Sixteen borderline hypertensive subjects had a cardiac index greater than the mean + 1SD of the normotensive control group (hyperkinetic subgroup) and 38 borderline hypertensive subjects had a cardiac index below that level (normokinetic subgroup). Minimal vascular resistance in the forearm and calf was assessed by plethysmography. Ambulatory 24-h blood pressure was recorded. RESULTS Subjects with hyperkinetic borderline hypertension had similar intra-arterial blood pressure levels to normokinetic borderline hypertensive subjects. Total peripheral resistance was lower in hyperkinetic borderline hypertensive than in normokinetic borderline hypertensive or normotensive control subjects. Hyperkinetic borderline hypertensive subjects had a significantly lower forearm minimal vascular resistance than normokinetic borderline hypertensive subjects. SBP and mean arterial blood pressure responses to stress were augmented in both borderline hypertensive subgroups. Hyperkinetic borderline hypertensive subjects also showed diastolic hyperreactivity in response to mental stress, in comparison both with normokinetic borderline hypertensive and with normotensive control subjects. During ambulatory blood pressure recording, hyperkinetic borderline hypertensive subjects had greater DBP and mean blood pressure increases from night to day than normotensive control and normokinetic borderline hypertensive subjects. CONCLUSIONS Borderline hypertension is characterized by pressor hyperreactivity to mental stress. In hyperkinetic borderline hypertensive subjects, stress hyperresponsiveness is also reflected by greater night-to-day blood pressure gradients during 24-h monitoring. Pressor hyperreactivity in hyperkinetic borderline hypertension is not explained by structural changes in the calf or forearm vasculature.