The purpose of this study was to pilot test the Comics for Health program, a theory-based nutrition and physical activity intervention for children. Twelve after-school programs were randomized to either a theory-based (n = 37) or a knowledge-based (n = 34 children) version of the intervention. Pretests, posttests, and 3-month follow-up tests were administered to evaluate the programmatic effects on body mass index percentile, obesity-related behaviors, and constructs of social cognitive theory. Both interventions found significant, yet modest effects for fruit and vegetable consumption (P < .005), physical activities (P < .004), and water and sugar-free beverage consumption (P < .001) and self-efficacy for fruit and vegetable consumption (P < .015) and physical activities (P < .009).