Background: The purpose of the present study was to examine whether the availability of intramural or extramural sports in secondary schools is associated with physical activity levels in youth throughout secondary school and at age 20. Methods: Eight hundred and eight adolescents from 10 secondary schools in Montreal, Canada, provided physical activity data every 3 months during the school year from ages 13 to 17, and again at age 20. School administrators completed questionnaires on the availability of intramural and extramural sports. Three-level general linear models were used to examine associations among the number of intramural and extramural sports, moderate and vigorous physical activity controlling for age, sex, body mass index, mother's education, and school-level socioeconomic status. Results: Regardless of whether or not they reported participating in intramural sports, adolescents in schools with more intramural sports engaged in 3.6 (p = 0.03) more total, and 1.3 (p = 0.03) more vigorous activities per week than those attending schools with fewer intramural sports. Number of extramural sports was not statistically significantly associated with physical activity, regardless of whether or not individual students participated. Conclusion: Providing more opportunities for intramural sports in secondary schools may be an effective strategy to help adolescents attain physical activity recommendations. (Contains 2 figures and 2 tables.)