BACKGROUND: Our objective was to evaluate the effects of environmental, policy, and social marketing interventions on physical activity and fat intake of middle school students on campus. DESIGN: Twenty-four middle schools were randomly assigned to intervention or control conditions. Baseline measures were collected in spring 1997, and interventions were conducted during the 1997-1998 and 1998-1999 school years SETTING/PARTICIPATION: The schools had mean enrollments of 1109, with 44.5% nonwhite students. Over 2 years, physical activity interventions were designed to increase physical activity in physical education classes and throughout the school day. Nutrition interventions were designed to provide and market low-fat foods at all school food sources, including cafeteria breakfasts and lunches, a la carte sources, school stores, and bag lunches. School staff and students were engaged in policy change efforts, but there was no classroom health education. MAIN OUTCOMES MEASURES: Primary outcomes were measured by direct observation and existing records. RESULTS: Randomized regression models (N =24 schools) revealed a significant intervention effect for physical activity for the total group (p <0.009) and boys (p <0.001), but not girls (p <0.40). The intervention was not effective for total fat (p <0.91) or saturated fat (p <0.79). Survey data indicated that the interventions reduced reported body mass index for boys (p <0.05). CONCLUSIONS: Environmental and policy interventions were effective in increasing physical activity at school among boys but not girls. The interventions were not effective in reducing fat intake at school. School environmental and policy interventions have the potential to improve health behavior of the student population, but barriers to full implementation need to be better understood and overcome.