Blood pressure and plasma catecholamine levels were followed up for a total of 7 years in 26 subjects with borderline hypertension and in 24 normotensive subjects. During the observation period, five subjects from the borderline hypertensive group became definitely hypertensive, 10 remained in the borderline hypertensive range and in 10, blood pressure fell below borderline hypertensive levels. Of the original 24 normotensive subjects, 15 remained in the normotensive range. In seven, blood pressure rose, but did not reach borderline hypertensive levels. Three subjects were lost to follow-up. In contrast with the subjects who remained normotensive and borderline hypertensive, those who became definitely hypertensive had consistently high plasma noradrenaline concentrations at rest. Plasma adrenaline levels and plasma noradrenaline concentrations during exercise did not differ, however. The study suggests that borderline hypertensive subjects who develop definite hypertension within a few years have consistently enhanced sympathetic activity.