Background: The National High Blood Pressure Education Program (NHBPEP) guidelines define hypertension (HT) in children and adolescents as blood pressure (BP) measures above the 95th percentile on three consecutive clinic visits. In contrast, the Expert Panel of Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents (Expert Panel) clinical practice guidelines define HT as the average of three consecutive BP measures above the 95th percentile. Here we quantify the impact of these two different case definitions of HT on occurrence of HT in a defined population of children and adolescents. Methods: Study subjects were a cohort of 117,329 pediatric primary care patients, drawn from three large, geographically dispersed health systems. Subjects were 3-17 years old at entry between January 1, 2007 and December 31, 2010. Subjects having an elevated initial BP were not excluded, nor were subjects having BP diagnosis codes at baseline or follow up. We estimated the period prevalence rate of HT using NHBPEP and Expert Panel definitions, applying a rolling window to the longitudinal BP measurements to consider each successive block of three BP measures. Analyses were performed separately for children (3-11 years) and adolescents (12-17 years). Data were analyzed by Poisson regression to estimate annual rates of HT. Results: Subjects were followed for an average of 2.1 years. HT defined by elevated BP >=95th percentile on 3 consecutive clinic visits occurred at a rate of 0.25%/year in children, and 0.42%/year in adolescents. HT defined as the average BP >=95th percentile from 3 successive clinic visits occurred at higher rates; 0.49%/year in children (p=<0.00005) and 0.75%/year in adolescents (p=<0.00005). For subjects with HT defined by elevated BP >=95th percentile on 3 consecutive clinic visits, the average time between elevated measurements was 19 weeks (s.d. 25 weeks). Conclusions: HT rates in children and adolescents were twice as high when using an average of 3 consecutive measures (Expert Panel method) as when using 3 consecutive hypertensive levels (NHBPEP definition). The impact of these differences in HT rates on downstream risk of persistent HT and CV events later in life requires further investigation.