The Joint National Committee on High Blood Pressure identified a new category of blood pressure in adults termed prehypertension. Our objective was to determine the long-term risk of cardiovascular diseases associated with this new category in a well-defined cohort of adults. We evaluated the association of prehypertension (120 to 139/80 to 89 mm Hg) and hypertension (>140/90 mm Hg) with the incidence of atherothrombotic brain infarction (ABI), all strokes, myocardial infarction (MI), and coronary artery disease (CAD) using pooled repeated measures and Cox proportional hazards analyses during follow-up after adjusting for age, gender, obesity, diabetes mellitus, hypercholesterolemia, cigarette smoking, and study period in a cohort of 5181 persons who participated in the Framingham Study. Among the 11,116 person observations with a mean follow-up period of 9.9+/-1.0 years, prehypertension was not associated with an increased risk for ABI (relative risk [RR], 2.2; 95% CI, 0.5 to 9.3). Among the 11,802 person observations with a mean follow-up period of 9.7+/-1.5 years, prehypertension was associated with an increased risk for MI (RR, 3.5; 95% CI, 1.6 to 7.5). Prehypertension was also associated with an increased risk of CADs among the 11,570 person observations (RR, 1.7; 95% CI, 1.2 to 2.4). Prehypertension appears to be associated with an increased risk of MI and CAD but not stroke. Further studies are required to confirm the anticipated benefits of identifying and intervening in persons with prehypertension.