Recent research indicates that recommended amounts of physical activity suggested for health benefits are rarely met in high-school–age adolescents. A pilot study was conducted to investigate the effects of a classroom health-education–based curriculum intervention on the physical activity of high school students. A within-group research design was used on data from a sample of ninth grade boys and girls (N = 104) who received six classroom health education lessons over 5 weeks based on social cognitive theory. The lessons focused on improvements in the theory-based psychological variables of mood, body satisfaction, physical self-concept, and exercise self-efficacy. Mixed-model repeated-measures ANOVAs indicated an overall statistically significant increase in physical activity, with no significant difference by sex. There were no statistically significant changes found in mood, body satisfaction, physical self-concept, or exercise self-efficacy. Results from a multiple regression analysis indicated that changes in the psychological variables tested explained 6% of the variance in physical activity change, which was not statistically significant. Results from this study may help researchers, school administrators, classroom health teachers, and curriculum developers better understand the role of the psychological factors of mood, body satisfaction, physical self-concept, and exercise self-efficacy in adolescent physical activity behavior and assist in the design of future school-based interventions.