The prevalence of masked hypertension (out-of-clinic daytime systolic/diastolic blood pressure (SBP/DBP) ≥135/85 mm Hg on ambulatory blood pressure monitoring [ABPM] among adults with clinic SBP/DBP <140/90 mm Hg) is high. It is unclear who should be screened for masked hypertension. The authors derived a clinic blood pressure (CBP) index to identify populations for masked hypertension screening. Index cut points corresponding to 75% to 99% sensitivity and prehypertension were evaluated as ABPM testing criterion. In a derivation cohort (n=695), the index was clinic SBP+1.3*clinic DBP. In an external validation cohort (n=675), the sensitivity for masked hypertension using an index ≥190 mm Hg and ≥217 mm Hg and prehypertension status was 98.5%, 71.5%, and 82.5%, respectively. Using National Health and Nutrition Examination Survey data (n=11,778), the authors estimated that these thresholds would refer 118.6, 44.4, and 59.3 million US adults, respectively, to ABPM screening for masked hypertension. In conclusion, the CBP index provides a useful approach to identify candidates for masked hypertension screening using ABPM.