The objective of this study was to evaluate the impact of a school-based intervention (Program X) incorporating pedometers and e-mail support on physical activity, sedentary behavior, and healthy eating in adolescents. A randomized control trial was used to evaluate the impact of the Program X intervention. Six schools (N = 124 participants; mean age 14.1 +/- .8 years) were randomized to intervention or control conditions for the 6-month study period. Objectively recorded physical activity (mean steps/day), self-reported sedentary behavior, and dietary habits were measured at baseline and at 6-month follow-up and intervention effects were assessed using repeated-measures analysis of variance and chi(2) tests. Participants in the intervention group increased their step counts by 956 +/- 4107 steps/day (boys) and 999 +/- 1999 (girls). Repeated-measures analysis of variance revealed significant group-by-time interactions for boys (F = 7.4, p = .01, d = .80) and girls (F = 29.6, p <.001, d = 1.27) for mean steps/day. The intervention significantly decreased the number of energy-dense/low-nutrient snacks consumed by boys (chi(2) = 4.0, p = .043) and increased the number of fruit serves among girls (chi(2) = 4.8, p = .028). The intervention did not have a statistically significant effect on sedentary behavior. A school-based intervention incorporating physical activity monitoring using pedometers and e-mail support was successful in promoting physical activity and selected healthy eating behaviors in adolescent boys and girls.