There is little national research on longitudinal patterns of physical activity and sedentary behavior in ethnically diverse teens as they transition to adulthood. Longitudinal questionnaire data from U.S. adolescents enrolled in Wave I (1994-1995) and Wave III (2001) of the National Longitudinal Study of Adolescent Health (n =13,030) were analyzed in January 2004. Incidence, reversal, and maintenance of achieving five or more weekly bouts of moderate to vigorous physical activity (MVPA) and ≤14 hours of weekly TV and video viewing, computer/video game use (screen time) were assessed. Multinomial logistic regression models examined the likelihood of achieving five or more weekly sessions of MVPA week and ≤14 hours screen time per week as an adolescent and/or young adult, controlling for household income, parental education, age of adolescent, and seasonality. Of those achieving five or more weekly sessions of MVPA and ≤14 hours of weekly screen time as adolescents, few continued to achieve these favorable amounts of activity (4.4%) and screen time (37.0%) as adults. More failed to maintain these favorable amounts of activity (31.1%) and screen time (17.3%) into adulthood. Black versus white females were more likely to maintain favorable amounts of activity from adolescence to adulthood (odds ratio [OR]=3.09; 95% confidence interval [CI]=1.49-6.42), while black males (OR=1.50; CI=1.05-2.14) and females (OR=2.00; CI=1.40-2.87) were more likely than whites to maintain less (versus more) favorable screen time hours. The vast majority of adolescents do not achieve five or more bouts of moderate physical activity per week, and continue to fail to achieve this amount of activity into adulthood.