This paper presents findings on the relationship between high school sport participation and involvement in sport as adults. The data are provided by a survey of a large representative national sample of adult Canadians. For different age subgroups among women and men, we tested the school sport experiences hypothesis that sport involvement during the high school years contributes to later adult involvement in sport. The measurement of sport involvement in the high school years is concerned with intramural and inter-school activities. Adult sport activity has three measures: sport involvement per se, involvement in an organized setting, and competitive involvement. The results are consistent with the school experiences hypothesis. High school sport involvement, for inter-school sport activities, is a comparatively strong predictor of adult sport involvement. The effects of high school involvement persist after controlling for correlated social background factors. Moreover, the effects of school sport experiences hold across age and gender subgroups. Although diminished with temporal distance from the high school years, the effects of high school involvement nonetheless extend even to respondents aged 40-59 (i.e., those approximately 22 to 42 years beyond their school years) among both genders. Interpretations of the results are discussed.