Objective: The purpose of this longitudinal study was to examine changes in physical activity behavior, psychological factors and interpersonal influences associated with exercise behavior during the transition from secondary school to high school in Sanandaj, Iran. Design: Data were collected from students in 2006 (N = 1,073), of whom 844 provided follow-up data in 2009. Setting: The students were in high school in 2009 (51% male) with a mean age of 16.42 + 1.73 years. Method: Physical activity-related cognitions, interpersonal influences, and physical activity behavior were measured using self-report questionnaires. To assess the effects of time and sex on psychosocial factors, analysis of variances (ANOVAs) were conducted. The relative stability of physical activity psychosocial correlates from wave 1 through wave 2 was assessed using Pearson product-moment correlations. Results: There were significant differences according to sex across time for physical activity, psychological variables and interpersonal influences associated with exercise activities. The girls were less active than boys at both time points. Girls reported lower self-efficacy and perceived more barriers and fewer perceived benefits for physical activity over time. Interpersonal influences on activity were more stable for girls than boys. Conclusion: The results offer evidence for decreasing physical activity in boys and girls and sex differences in cognitive variables and interpersonal influences. This is important for understanding how to address the problem of physical inactivity among Iranian youth. (Contains 4 tables.)