Awareness of hypertension increases blood pressure and sympathetic responses to cold pressor test. The present study was aimed at examining the effects of awareness of hypertension on blood pressure and sympathetic responses to the cold pressor test. Nineteen-year-old men with similarly elevated mean blood pressure at a medical screening, but without knowledge of this, were randomized into two groups. The first group (n = 16) was sent a letter saying that their pressure was too high, and the second (n = 13) was sent a neutral letter. Information increased mean blood pressure both after 15 min sitting, by an average of 11.5 mm Hg (P less than .01), and after 30 min supine rest, by an average of 4.5 mm Hg (P less than .05). Changes in heart rate (8.4 +/- 2.4 v 1.9 +/- 1.7 beats/min) and plasma epinephrine (0.11 +/- 0.04 v 0.01 +/- 0.03 nmol/L) during execution of a cold pressor test were significantly greater in the informed group (P less than .05). Plasma dopamine was lower in the informed group (P less than .05). Thus, psychological stress caused by the awareness of hypertension may increase blood pressure and sympathetic responses to a provocative maneuver. Ideally, studies on sympathetic function in essential hypertension should be undertaken on subjects unaware of their blood pressure status. 