© 2017 The Authors. Background-Clinical guidelines recommend using predicted atherosclerotic cardiovascular disease (ASCVD) risk to inform treatment decisions. The objective was to compare the contribution of changes in modifiable risk factors versus aging to the development of high 10-year predicted ASCVD risk. Methods and Results-A prospective follow-up was done of the Jackson Heart Study, an exclusively black cohort at visit 1 (2000-2004) and visit 3 (2009-2012). Analyses included 1115 black participants without high 10-year predicted ASCVD risk (<7.5%), hypertension, diabetes mellitus, or ASCVD at visit 1. We used the Pooled Cohort equations to calculate the incidence of high (≥7.5%) 10-year predicted ASCVD risk at visit 3. We recalculated the percentage with high 10-year predicted ASCVD risk at visit 3 assuming each risk factor (age, systolic blood pressure, antihypertensive medication use, diabetes mellitus, smoking, total and high-density lipoprotein cholesterol), one at a time, did not change from visit 1. The mean age at visit 1 was 45.2±9.5 years. Overall, 30.9% (95% CI 28.3-33.4%) of participants developed high 10-year predicted ASCVD risk. Aging accounted for 59.7% (95% CI 54.2-65.1%) of the development of high 10-year predicted ASCVD risk compared with 32.8% (95% CI 27.0-38.2%) for increases in systolic blood pressure or antihypertensive medication initiation and 12.8% (95% CI 9.6-16.5%) for incident diabetes mellitus. Among participants <50 years, the contribution of increases in systolic blood pressure or antihypertensive medication initiation was similar to aging. Conclusions-Increases in systolic blood pressure and antihypertensive medication initiation are major contributors to the development of high 10-year predicted ASCVD risk in blacks, particularly among younger adults.