OBJECTIVE: To prospectively evaluate the relationship between household income, children's cortisol, and body mass index (BMI) trajectories over a 3-year period in early childhood. STUDY DESIGN: Household income, child hair cortisol levels, and BMI were measured at baseline, 12-, 24-, and 36-month follow-up visits in the Now Everybody Together for Amazing and Healthful Kids (NET-Works) Study (n = 534, children ages 2-4 years, and household income <$65 000/year at baseline). Relationships were examined between very low household income (<$25 000/year) at baseline, income status over time (remained <$25 000/year or had increasing income), cortisol accumulation from hair samples, and BMI percent of the 95th percentile (BMIp95) trajectories using adjusted linear growth curve modeling. Households with baseline income between $25 000 and $65 000/year were the reference group for all analyses. RESULTS: Children from very low-income households at baseline had annual changes in BMIp95 that were higher (P < .001) than children from reference group households (0.40 vs -0.62 percentage units/year). Annual increases in BMIp95 were also greater among children from households that remained very low income (P < .01, .34 percentage units/year) and among those with increasing income (P = .01, .51 percentage units/year) compared with the reference group (-0.61 percentage units/year). Children from households that remained very low income had higher hair cortisol accumulations (0.22 pg/mg, P = .02) than reference group children, whereas hair cortisol concentrations of children from households with increasing income (0.03 pg/mg) did not differ significantly from the reference group. Cortisol was not related to BMIp95. CONCLUSIONS: The economic circumstances of families may impact children's BMI trajectories and their developing stress systems, but these processes may be independent of one another.