Background: Understanding childhood obesity's root causes is critical to the creation of strategies to improve our children's health. We sought to define the association between childhood obesity and household income and how household income and childhood behaviors promote childhood obesity. Methods: We assessed body mass index in 109,634 Massachusetts children, identifying the percentage of children who were overweight/obese versus the percentage of children in each community residing in low-income homes. We compared activity patterns and diet in 999 sixth graders residing in 4 Michigan communities with varying annual household income. Results: In Massachusetts, percentage of overweight/obese by community varied from 9.6% to 42.8%. As household income dropped, percentage of overweight/obese children rose. In Michigan sixth graders, as household income goes down, frequency of fried food consumption per day doubles from 0.23 to 0.54 (P <.002), and daily TV/video time triples from 0.55 to 2.00 hours (P <.001), whereas vegetable consumption and moderate/vigorous exercise go down. Conclusions: The prevalence of overweight/obese children rises in communities with lower household income. Children residing in lower income communities exhibit poorer dietary and physical activity behaviors, which affect obesity. © 2012 Mosby, Inc.