Data were obtained from the Michigan Study of Adolescent Life Transitions. This longitudinal study began in 1983 with a cohort of sixth graders from 10 school districts in southeastern Michigan. A majority of the sample was White and came from working-class and middle-class families living in small industrial cities in the Detroit area. The study followed approximately 1,800 of these youth through 8 waves of data collection, beginning in the sixth grade (1983-1984) and continuing into 1996-97 when most were 24 to 25 years old. The analyses of the current study included more than 600 respondents who completed the survey items pertinent to sports and physical fitness from 3 waves of data (wave 1 at age 12, wave 6 at age 17, and wave 8 at age 25). The surveys were mailed to young adults. Data were obtained on family structure; parental education; family income; young adult marital status, education level, parental status, and socioeconomic status; sports participation in childhood, adolescence, and young adulthood; and fitness participation in young adulthood. The study confirmed the findings of earlier studies that childhood and adolescent sports participation shaped individuals' participation in sports and fitness activities in later life. Future research should examine the determinants of participation in sports and fitness activities for adolescents and adults and the relationship between them. Applied research should identify what motivates children and adolescents to become involved and stay involved in organized sports and what causes them to leave. In addition, future research is needed to identify the characteristics of quality sports experiences and what other nonathletic skills and competencies are fostered through such opportunities. 4 tables and 35 references