BACKGROUND: Evidence consistently shows children in the United States gain 3 to 5 times more weight during summer vacation (~2.5 months) compared to the 9-month school year. The purpose of this study is to examine within-child differences in 4 obesogenic behaviors (physical activity [PA], sedentary/screen-time, diet, and sleep) during school versus summer. METHODS: We used a repeated-measures within-subjects design. Children (N = 30 mean age = 8.2 years; 57% female; 37% overweight/obese; 100% African American) wore accelerometers on the nondominant wrist for 24 hr/d over 9 consecutive days during school and summer of 2016 to capture PA, sedentary time, and sleep. Parents completed a daily diary to report bed/wake times, diet, and screen-time of their child each day. Mixed-effect models compared summer and school behaviors. RESULTS: Children spent more time sedentary (69% vs 67% of wake wear time), less time in light PA (25% vs 23% of wake wear time), had higher screen-time (242 vs 123 minutes/day), slept longer (428 vs 413 minutes/night), and consumed more sugar-based foods (6 days vs 2.5 days/week) and fruit (7 days vs 4.7 days/week) during summer compared to school (p < 0.05). CONCLUSION: Initial evidence suggests children are displaying multiple unfavorable obesogenic behaviors during summer compared to school that may contribute to accelerated weight gain during summer.