This article describes the process of setting up a 3-year, school-based after-school physical activity intervention in elementary schools. The primary aim of the study is to determine whether adiposity and fitness will improve in children who are exposed to a fitogenic versus an obesogenic environment. Eighteen schools were randomized to the control (obesogenic) or intervention (fitogenic) group. The study design, program components, and evaluation of the intervention are described in detail. The intervention consists of (a) academic enrichment, (b) a healthy snack, and (c) physical activity in a mastery-oriented environment. Successful implementation would show the feasibility of schools' being able to provide a fitogenic environment. Significant differences between the groups would provide evidence that a fitogenic environment after school has positive health benefits. If feasibility and efficacy are demonstrated, implementing an after-school program like this one in elementary schools could play a major role in preventing and reducing childhood obesity. (author abstract)