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Background: We examined prevalence, incidence, and trajectory of obesity from kindergarten through grade 8 in one of the first states to implement annual surveillance. Methods: Participants included 16,414 children enrolled in kindergarten in Arkansas in 2004 with complete body mass index (BMI) measurements in kindergarten and eighth grade. Repeated measures of weight status were entered in multiple linear and logistic regression models with demographics and family poverty status. Results: The prevalence of obesity (BMI = 95th percentile) was lowest in kindergarten (14.9%), with subsequent incidence rates consistent at 4%. Prevalence and incidence peaked in eighth grade (24.5% and 4.9%, respectively), with 33.8% of children measuring obese at least once by eighth grade. Kindergarten obesity was a significant predictor of eighth grade obesity (odds ratio, 17.5; 95% confidence interval, 15.8-19.3). We found statistically significant 3-way interactions for sex, race, and time, suggesting unique patterns for Hispanic boys and black girls. Conclusions: Our study documents unique longitudinal patterns of obesity from kindergarten through eighth grade that expand our understanding of risk. It demonstrates the value of public school health systems that collect routine administrative data about student BMI that is integrated with education records to foster program and policy discussions.