Ambulatory blood pressure (BP) monitoring provides a more precise measure of BP status than clinic BP and is currently recommended in the evaluation of high BP in children and adolescents. However, ambulatory BP monitoring may not always be available. Our aim was to determine the clinic BP percentile most likely to predict ambulatory hypertension. We evaluated clinic and ambulatory BP in 247 adolescents (median age, 15.7 years; 63% white; 54% male). Clinic BP percentile (based on the fourth report and the 2017 American Academy of Pediatrics clinical practice guidelines) and ambulatory BP status (normal versus hypertension) were determined by age-, sex-, and height-specific cut points. Sensitivity and specificity of different clinic BP percentiles and cutoffs to predict ambulatory hypertension were calculated. Forty (16%) and 67 (27%) patients had systolic hypertension based on the fourth report and the 2017 guidelines, respectively, whereas 38 (15%) had wake ambulatory systolic hypertension. The prevalence of ambulatory wake systolic hypertension increased across clinic systolic BP percentiles, from 3% when clinic systolic BP was <50th percentile to 41% when ≥95th percentile. The 2017 guidelines' 85th systolic percentile had similar sensitivity (86.8%) and better specificity (57.4% versus 48.1%) than elevated BP (≥90th percentile or ≥120 mm Hg) to diagnose ambulatory hypertension. When evaluating adolescents for hypertension, 2017 guidelines' clinic systolic 85th percentile may be the optimal threshold at which to perform ambulatory BP monitoring.