Parents are believed to play a role in influencing their children's health behaviours. This longitudinal study of two generations (parents and their children) examined associations between parents' self-reported leisure-time physical activity changes and the self-reported physical activity changes of their offspring in a sample of 557 adolescents over an 8 year period (from 13 to 21 years of age). The results revealed only weak and non-existent associations between changes in parents' physical activity and changes in adolescent physical activity from 13 to 21 years of age. The findings did not support the hypothesis that adolescents' leisure-time physical activity covaried with their parents' leisure-time physical activity over time. This may mean that parental physical activity is not transmitted to their children to the degree that is often believed.