The goal of this study was to determine if Head Start participation is associated with abstract healthy changes in BMI. Methods: The sample included children participating in Head Start between 2005 and 2013 and children from 2 comparison groups drawn from a Michigan primary care health system: 5405 receiving Medicaid and 19 320 not receiving Medicaid. Change in BMI z score from the beginning to the end of each of 2 academic years and the intervening summer was compared between groups by using piecewise linear mixed models adjusted for age, gender, and race/ethnicity. Results: The total sample included 43 748 children providing 83 239 anthropometric measures. The Head Start sample was 64.9% white, 10.8% black, and 14.4% Hispanic; 16.8% of the children were obese and 16.6% were overweight at the initial observation. Children who entered Head Start as obese exhibited a greater decline in the BMI z score during the first academic year versus the comparison groups ([beta] = -0.70 [SE: 0.05] vs -0.07 [0.08] in the Medicaid group [P .001] and -0.15 [SE: 0.05] in the Not Medicaid group [P .001]); patterns were similar for overweight children. Head Start participants were less obese, less overweight, and less underweight at follow-up than children in the comparison groups. Conclusions: Preschool-aged children with an unhealthy weight status who participated in Head Start had a significantly healthier BMI by kindergarten entry age than comparison children in a primary care health system (both those receiving and those not receiving Medicaid). (author abstract)