Importance: Little is known regarding the association between level of blood pressure (BP) in young adulthood and cardiovascular disease (CVD) events by middle age. Objective: To assess whether young adults who developed hypertension, defined by the 2017 American College of Cardiology (ACC)/American Heart Association (AHA) BP guideline, before age 40 years have higher risk for CVD events compared with those who maintained normal BP. Design, Setting, and Participants: Analyses were conducted in the prospective cohort Coronary Artery Risk Development in Young Adults (CARDIA) study, started in March 1985. CARDIA enrolled 5115 African American and white participants aged 18 to 30 years from 4 US field centers (Birmingham, Alabama; Chicago, Illinois; Minneapolis, Minnesota; and Oakland, California). Outcomes were available through August 2015. Exposures: Using the highest BP measured from the first examination to the examination closest to, but not after, age 40 years, each participant was categorized as having normal BP (untreated systolic BP [SBP] <120 mm Hg and diastolic BP [DBP] <80 mm Hg; n = 2574); elevated BP (untreated SBP 120-129 mm Hg and DBP <80 mm Hg; n = 445); stage 1 hypertension (untreated SBP 130-139 mm Hg or DBP 80-89 mm Hg; n = 1194); or stage 2 hypertension (SBP ≥140 mm Hg, DBP ≥90 mm Hg, or taking antihypertensive medication; n = 638). Main Outcomes and Measures: CVD events: fatal and nonfatal coronary heart disease (CHD), heart failure, stroke, transient ischemic attack, or intervention for peripheral artery disease (PAD). Results: The final cohort included 4851 adults (mean age when follow-up for outcomes began, 35.7 years [SD, 3.6]; 2657 women [55%]; 2441 African American [50%]; 206 taking antihypertensive medication [4%]). Over a median follow-up of 18.8 years, 228 incident CVD events occurred (CHD, 109; stroke, 63; heart failure, 48; PAD, 8). CVD incidence rates for normal BP, elevated BP, stage 1 hypertension, and stage 2 hypertension were 1.37 (95% CI, 1.07-1.75), 2.74 (95% CI, 1.78-4.20), 3.15 (95% CI, 2.47-4.02), and 8.04 (95% CI, 6.45-10.03) per 1000 person-years, respectively. After multivariable adjustment, hazard ratios for CVD events for elevated BP, stage 1 hypertension, and stage 2 hypertension vs normal BP were 1.67 (95% CI, 1.01-2.77), 1.75 (95% CI, 1.22-2.53), and 3.49 (95% CI, 2.42-5.05), respectively. Conclusions and Relevance: Among young adults, those with elevated blood pressure, stage 1 hypertension, and stage 2 hypertension before age 40 years, as defined by the blood pressure classification in the 2017 American College of Cardiology/American Heart Association (ACC/AHA) guidelines, had significantly higher risk for subsequent cardiovascular disease events compared with those with normal blood pressure before age 40 years. The ACC/AHA blood pressure classification system may help identify young adults at higher risk for cardiovascular disease events.