Technical Abstract: The objective was to evaluate shifts across BMI categories and associated changes in cardiometabolic risk factors over 2.5 years in an ethnically diverse middle school sample. As part of HEALTHY, a multisite school-based study designed to mitigate risk for type 2 diabetes, 3993 children participated in health screenings at the start of sixth and end of eighth grades. Assessments included anthropometric measures, blood pressure, and glucose, insulin, and lipids. Students were classified as underweight, healthy weight,overweight, obese, or severely obese. Mixed models controlling for school intervention status and covariates were used to evaluate shifts in BMI category over time and the relation between these shifts and changes in risk factors. At baseline, students averaged 11.3 (60.6) years; 47.6% were boys, 59.6% were Hispanic, and 49.8% were overweight or obese.Shifts in BMI category over time were common. For example, 35.7% of youth who were overweight moved to the healthy weight range, but 13% in the healthy weight range became overweight. BMI shifts were not associated with school intervention condition, household education,or youth gender, race/ethnicity, pubertal status, or changes in height. Increases in BMI category were associated with worsening of cardiometabolic risk factors, and decreases were associated with improvements. Boys who increased BMI category were more vulnerable to negative risk factor changes than girls. In conclusion, there are substantial shifts across BMI categories during middle school that are associated with clinically meaningful changes in cardiometabolic risk factors. Programs to promote decreases in BMI and prevent increases are clearly warranted.