Background: High blood pressure in middle age is a well-established risk factor for cardiovascular disease, but the consequences of low-level elevations during young adulthood are unknown. Objective: To measure the association between prehypertension exposure before age 35 years and coronary calcium later in life. Design: Prospective cohort study. Setting: Four communities in the United States. Participants: Black and white men and women age 18 to 30 years recruited for the CARDIA (Coronary Artery Risk Development in Young Adults) Study in 1985 through 1986 who were without hypertension before age 35 years. Measurements: Blood pressure trajectories for each participant were estimated by using measurements from 7 examinations over the course of 20 years. Cumulative exposure to blood pressure in the prehypertension range (systolic blood pressure of 120 to 139 mm Hg, or diastolic blood pressure of 80 to 89 mm Hg) from age 20 to 35 years was calculated in units of mm Hg-years (similar to pack-years of tobacco exposure) and related to the presence of coronary calcium measured at each participant's last examination (mean age, 44 years [SD, 4]). Results: Among 3560 participants, the 635 (18%) who developed prehypertension before age 35 years were more often black, male, overweight, and of lower socioeconomic status. Exposure to prehypertension before age 35 years, especially systolic prehypertension, showed a graded association with coronary calcium later in life (coronary calcium prevalence of 15%, 24%, and 38% for 0, 1 to 30, and >30 mm Hg-years of exposure, respectively; P < 0.001). This association remained strong after adjustment for blood pressure elevation after age 35 years and other coronary risk factors and participant characteristics. Limitation: Coronary calcium, although a strong predictor of future coronary heart disease, is not a clinical outcome. Conclusion: Prehypertension during young adulthood is common and is associated with coronary atherosclerosis 20 years later. Keeping systolic pressure below 120 mm Hg before age 35 years may provide important health benefits later in life. © 2008 American College of Physicians.