The purpose of this study was to identify distinct trajectories of smoking behavior during a period extending from adolescence (mean age = 14 years) to young adulthood (mean age = 26 years) among African American and Puerto Rican adolescents/young adults, to examine ethnic and gender differences in group membership, and to assess the ability of peer and parental smoking to distinguish among trajectory groups. A community-based sample of 451 African American and Puerto Rican adolescents was interviewed four times during adolescence and in early adulthood, covering a span of 12 years. For both ethnic/racial groups, four distinct trajectories were identified: Nonsmokers, maturing-out smokers, late-starting smokers, and early-starting continuous smokers. Compared with Puerto Ricans, African Americans were over-represented in the nonsmoking group, whereas Puerto Ricans were over-represented in the early-starting continuous group. Females were more likely than males to be early-starting continuous smokers than late starters. Adolescents who were exposed to peer and parental smoking in early adolescence were more likely to belong to trajectory groups characterized by higher levels of smoking. These findings show that exposure to peer and parental smoking in early adolescence constitutes a risk factor for engaging in elevated levels of smoking behavior at an early age and for continued smoking into adulthood for urban African Americans and Puerto Ricans. To be most effective, smoking prevention programs should address peer group and family influences on adolescent smoking.