Influence from parents and friends on adolescents' self-reported level of leisure-time physical activity was examined through measures of (a) perceived leisure-time physical activity of parents and best friend, (b) perceived direct support for physical activity from parents and friends, (c) direct help from parents in exercising vigorously, and (d) perceived value of physical activity of parents and friends. The present analysis included 904 students in western Norway (M age 13.3 years, SD 0.3) who took part in a health behavior survey, which included questions regarding physical activity. Students completed a confidential questionnaire at school. All four measures of influence were positively related to students' activity levels. None of the examined measures was clearly stronger than the other. Parental and peer physical activity level and parental and peer support for physical activity seem to influence the reported physical activity level of the respondents. The findings indicate that, by serving as models and supporters, significant others have an important impact in promoting physical activity in young adolescents.