Semiparametric group-based mixture modeling was used with data from an adolescent school sample (N = 1205) for three purposes. First, five trajectory groups were identified to characterize different patterns of change in the frequency of marijuana use across four waves of assessment during adolescence. These trajectory groups were labeled Abstainers, Experimental Users, Decreasers, Increasers, and High Chronics. Second, trajectory group comparisons were made across eight adolescent risk factors to determine distinctive predictors of the trajectory groups. Findings indicated, for example, that the High Chronic group, relative to the other trajectory groups, had higher levels of delinquency, lower academic performance, more drug using friends, and more stressful life events. Third, adolescent trajectory group comparisons were made across 10 risk behaviors in young adulthood (average subject age = 23.5 years) and the occurrence of psychiatric and substance abuse disorders. Findings indicated some consistency across adolescence to young adulthood with regard to risk factors, and specificity with regard to the prediction of disorders. Adolescent trajectory group membership was significantly associated in young adulthood with cannabis and alcohol disorders but not with major depressive disorders or anxiety disorders.