Background The relationship between changes in family socioeconomic status (SES) and the development of obesity in childhood is unknown. The purpose of this research was to investigate the relationship between growth and two measures of SES change in childhood: family income trajectory and early-life food insecurity, and whether SES modified the relationship between the school nutrition and physical activity (N&PA) environment and growth during middle school, 6th to 8th grade. Methods This longitudinal research employed a birth cohort (n=595) located in rural New York State, followed from birth to 15 years. Data were collected through an audit of medical records, mailed questionnaires, and an assessment of the middle school N&PA environments. Family income and body mass index (BMI) z-score trajectories were created using latent-class modeling techniques to group children based on similar trends across time. Linear mixed models were used to estimate rate change in BMI zscore. Results 1. Children with poorer income trajectories were more likely to be in overweight and obese trajectories. Children who were persistently low-income were more likely to be in the overweight-stable BMI trajectory, and downwardly mobile children were more likely to be in the obese BMI trajectory. 2. The association between food insecurity and growth status varied across time. Foodinsecure children had a lower estimated BMI z-score in early childhood compared to food-secure children, but their elevated rate of growth during childhood resulted in a higher estimated BMI z-score by the age of 15 compared to food-secure children. 3. The association between the school environment and change in BMI z-score depended on income trajectory. Specifically, reductions in BMI z-score were associated with better physical education and general physical activity promotion environments among adolescents with unstable and persistent low-income trajectories. Conclusion Low SES trajectories and food insecurity were positively associated with the development of overweight and obesity in complex ways across childhood. Depending on an adolescent's income trajectory, better middle school environments for physical activity were associated with decreased obesity risk.