© 2019 Lippincott Williams and Wilkins. All rights reserved. Objective:To evaluate the association of sex with masked hypertension, defined by out-of-clinic hypertension based on ambulatory blood pressure monitoring (ABPM) among adults without hypertension based on blood pressure (BP) measured in the clinic, after adjusting for potential confounders.Methods:We evaluated sex differences in the prevalence of masked hypertension and the difference between awake, or alternatively 24-h, ambulatory BP and clinic BP using multivariable adjusted models among 658 participants who underwent 24-h ABPM and had clinic SBP/DBP less than 140/90 mmHg during the Year 30 Exam of the Coronary Artery Risk Development in Young Adults study.Results:The mean age ± standard deviation (SD) of the participants was 54.8 ± 3.7 years, 58.4% were women, and 58.2% were black. The prevalence of any masked hypertension was 37.5% among women and 60.6% among men. In a model including adjustment for demographics, cardiovascular risk factors, antihypertensive medication, and clinic BP, the prevalence ratios (95% confidence intervals) comparing men versus women were 1.39 (1.18-1.63) for any masked hypertension, and 1.60 (1.28-1.99), 1.71 (1.36-2.15), and 1.40 (1.13-1.73) for masked awake, 24-h and asleep hypertension, respectively. In a fully adjusted model, the differences between mean awake ambulatory BP and clinic BP were 2.75 [standard error (SE) 0.92] mmHg higher for SBP and 3.61 (SE 0.58) mmHg higher for DBP among men compared with women.Conclusion:The prevalence of masked hypertension on ABPM was high in both men and women. Male sex was an independent predictor of masked hypertension.