© 2019 Wolters Kluwer Health, Inc. All rights reserved. Background:The 2017 American College of Cardiology/American Heart Association (ACC/AHA) blood pressure (BP) guideline lowered the BP thresholds for defining hypertension compared with the Seventh Report of the Joint National Committee (JNC7) guideline.Methods:We analyzed clinic and ambulatory BP monitoring data from 717 Coronary Artery Risk Development in Young Adults study participants and compared the prevalence of clinic and out-of-clinic BP phenotypes using thresholds from the 2017 ACC/AHA and JNC7 guidelines.Results:Among participants not taking antihypertensive medication and according to the JNC7 and 2017 ACC/AHA guidelines, 11.1 and 30.1% of participants had clinic hypertension, 37.5 and 57.9% had awake hypertension, 35.7 and 58.1% had asleep hypertension, and 35.7 and 58.6% had 24-h hypertension, respectively. According to the JNC7 and 2017 ACC/AHA guideline definitions, 1.9 and 3.2% had white-coat hypertension, 28.2 and 31.0% had masked hypertension and 9.3 and 26.9% had sustained hypertension, respectively. Among participants taking antihypertensive medication and when defined using the JNC7 and 2017 ACC/AHA guideline BP thresholds, 18.6 and 45.3% had uncontrolled clinic BP, 48.1 and 62.5% had uncontrolled awake BP, 48.1 and 70.2% had uncontrolled asleep BP and, 47.7 and 65.3% had uncontrolled 24-h BP, respectively. Using JNC7 and 2017 ACC/AHA guideline BP thresholds, the prevalence was 1.4 and 5.2% for white-coat effect, 30.9 and 22.5% for masked uncontrolled hypertension, and 17.2 and 40.0% for sustained uncontrolled BP, respectively.Conclusion:The 2017 ACC/AHA guideline results in a substantially higher prevalence of awake, asleep, 24-h, and sustained hypertension.