Background: Overweight and obesity among US children and adolescents is an important public health problem. Conditions associated with obesity, such as type 2 diabetes, hypertension, and hypercholesterolemia, are becoming more common in children. This study examined the relationship between changes in BMI percentile and incident prehypertension and hypertension in a cohort of children and adolescents. Methods: Study subjects were 23,578 patients, ages 3-17, with three or more outpatient primary care visits between 2007 and 2010 at HealthPartners Medical Group, Kaiser Permanente Colorado, or Kaiser Permanente Northern California. Data were extracted from electronic health records (EHR). Change in BMI was defined as: increase, decrease, stayed obese, stayed overweight, and stayed healthy weight using established BMI percentile cut-points. Incident prehypertension and hypertension were defined using blood pressures and diagnosis codes from the EHR. We used time-dependent Cox proportional hazards models to estimate the hazard of change in BMI percentile with incident prehypertension and hypertension. Results: Over a median 2.6 years follow-up, there were 7,232 cases of incident prehypertension, 148 diagnoses of incident hypertension, and 107 additional cases of incident hypertension based on blood pressure data from the EHR. Seventy-one prehypertensives went on to develop hypertension. Those who stayed obese, stayed overweight, and increased BMI had increased hazard of incident prehypertension (1.96, 1.39, and 1.49, respectively) and increased hazard of incident hypertension (3.61, 1.21, and 1.83, respectively) compared with those who stayed healthy weight. Conclusions: Persistently high BMI or increasing BMI over time was associated with pronounced increase in risk of both incident prehypertension and hypertension. Future research should examine factors associated with the development and recognition of hypertension.