Background: The increasing numbers of children or adolescents having elevated blood pressure (BP) is an important public health problem, but little information is available regarding patterns of care or resource use implications in this population. We established a data cohort of children and adolescents to study these relationships. Methods: We have implemented a longitudinal cohort study based on routinely collected electronic medical record data from 3 large health plans. The study will ultimately extend from calendar year 2007 through 2012. We have currently extracted subject data through 2009. We will continue to accrue new subjects through 2012 and to add data for existing subjects in future years. The source population consists of children and adolescents (3-17 yr) enrolled in the 3 health plans. Beginning in Jan 2007, age-eligible subjects enter the cohort at the time of the first clinic visit in which an eligible BP is measured (i.e., collected during an outpatient visit and associated with a corresponding height measure). Subjects are being followed forward in time with longitudinal data collection. Demographic, clinical, and administrative data include age, gender, race/ethnicity, BP, height, weight, diagnoses, medications, and cost of care. We have conducted preliminary analyses to demonstrate the accrual of subjects into the cohort over years 2007 through 2009, and describing the gender, age, and racial composition of the study population. Results: As of the end of 2009, the cohort included 330,880 children and adolescents. At entry, 58.1% were 3-11 years of age, whereas 41.9% were 12-17 years. Subjects were 50.2% female. Race was 41.0% white (combining Hispanics and non-Hispanics), 8.1% African-American, 12.8% Asian/Pacific Islander, 16.3% other, and 21.8% missing. Conclusions: We have created a large pediatric cohort of subjects using data collected from health plan electronic medical and administrative records at several HMORN sites. These de-identified data include longitudinal BP, BMI, and utilization data suitable to test hypotheses related to correlation of BP and BMI over time, the influence of BMI on BP, patterns of care provided to those with elevated BP and BMI, and the influence of elevated BP and BMI on resource use.