Abstract This study used the Interaction Model of Client Health Behavior (IMCHB) as a conceptual guide to explain the correlates of children's diet and physical activity and explore the relationships of sex with their diet and physical activity of the school-aged child. A descriptive correlational study was conducted on 371 fifth-grade students and their parents. Information on the family's demographics, health experience, social influence, and environmental resources was collected, as well as data on the children's intrinsic motivation, cognitive appraisal, and affective response to food/physical activity. Children's self-reports on diet and physical activity were collected, as were parents' self-reports on health habits. Food preferences and diet self-efficacy explained the most variance in diet behavior for girls and boys. Girls scored healthier on food preferences and diet self-efficacy than did boys, but no difference was detected in their diet behavior. Girls participated in more low-intensity physical activity, but boys participated in more high-intensity physical activity than did girls. Findings provide strong support for the use of the IMCHB to explain children's diet but weak support for the explanation of children's physical activity. Further study of additional factors predictive of physical activity is indicated.