Background/Aims: Identification of persistently elevated blood pressure (BP) in children can be challenging because BP measurements are subject to biological fluctuation and measurement error. The National High Blood Pressure Education Program 4th Working Group (NHBPEP) Guidelines define hypertension (HT) as BP that is =95th percentile for sex, age and height on 3 consecutive clinic measurements on different days. However, HT is a chronic condition, and it is not clear whether a definition incorporating multiple measures over a longer period of time would alter the prevalence compared to the present definition of HT. Methods: Subjects included all children and adolescents, 3 to 17 years old, who received care at a midwestern medical group and had three or more BP percentile measures taken between 1 January 2007 and 31 December 2010. All data were collected from an electronic medical record. We compared the prevalence of HT based on BP = 95th percentile on 3 consecutive visits with the prevalence based on the average of multiple BP percentile measurements over the previous 2 or previous 4 years. Results: Among the 22,173 children and adolescents in this cohort, HT prevalence differed according to the length of the measurement period and the classification method used. The NHBPEP definition of HT found 1.2% of children were hypertensive using 2 years of BP measurements, and 2.1% of children using 4 years of measurements. Averaging all measurements, 1.3% of children were classified as hypertensive using 2 years of BP measurements and 0.5% using 4 years of measurements. Conclusions: The 2 methods found similar prevalence of HT over 2 years of observation, but over 4 years, the averaging method resulted in 4 times fewer children with HT. We speculate that the current NHBPEP approach may over-diagnose HT and that longer periods of observation may provide a more valid estimate of BP status.