Masked hypertension, defined as nonelevated clinic blood pressure (BP) and elevated out-of-clinic BP may be an intermediary stage in the progression from normotension to hypertension. We examined the associations of out-of-clinic BP and masked hypertension using ambulatory BP monitoring with incident clinic hypertension in the Jackson Heart Study, a prospective cohort of blacks. Analyses included 317 participants with clinic BP <140/90 mm Hg, complete ambulatory BP monitoring, who were not taking antihypertensive medication at baseline in 2000 to 2004. Masked daytime hypertension was defined as mean daytime blood pressure ≥135/85 mm Hg, masked night-time hypertension as mean night-time BP ≥120/70 mm Hg, and masked 24-hour hypertension as mean 24-hour BP ≥130/80 mm Hg. Incident clinic hypertension, assessed at study visits in 2005 to 2008 and 2009 to 2012, was defined as the first visit with clinic systolic/diastolic BP ≥140/90 mm Hg or antihypertensive medication use. During a median follow-up of 8.1 years, there were 187 (59.0%) incident cases of clinic hypertension. Clinic hypertension developed in 79.2% and 42.2% of participants with and without any masked hypertension, 85.7% and 50.4% with and without masked daytime hypertension, 79.9% and 43.7% with and without masked night-time hypertension, and 85.7% and 48.2% with and without masked 24-hour hypertension, respectively. Multivariable-adjusted hazard ratios (95% confidence interval) of incident clinic hypertension for any masked hypertension and masked daytime, night-time, and 24-hour hypertension were 2.13 (1.51-3.02), 1.79 (1.24-2.60), 2.22 (1.58-3.12), and 1.91 (1.32-2.75), respectively. These findings suggest that ambulatory BP monitoring can identify blacks at increased risk for developing clinic hypertension.