OBJECTIVE: To determine how early puberty and peer deviance relate to trajectories of aggressive and delinquent behavior in early adolescence and whether these relationships differ by race/ethnicity. METHODS: In this longitudinal study, 2607 girls from 3 metropolitan areas and their parents were interviewed at ages 11, 13, and 16 years. Girls reported on their age of onset of menarche, best friend's deviant behavior, delinquency, and physical, relational, and nonphysical aggression. Parents provided information on family sociodemographic characteristics and girls' race/ethnicity. RESULTS: Sixteen percent of girls were classified as early maturers (defined by onset of menarche before age 11 years). Overall, relational and nonphysical aggression increased from age 11 to age 16, whereas delinquency and physical aggression remained stable. Early puberty was associated with elevated delinquency and physical aggression at age 11. The relationship with early puberty diminished over time for physical aggression but not for delinquency. Best friend's deviant behavior was linked with higher levels of all problem behaviors, but the effect lessened over time for most outcomes. Early puberty was associated with a stronger link between best friend's deviance and delinquency, suggesting increased vulnerability to negative peer influences among early-maturing girls. A similar vulnerability was observed for relational and nonphysical aggression among girls in the "other" racial/ethnic minority group only. CONCLUSIONS: Early puberty and friends' deviance may increase the risk of problem behavior in young adolescent girls. Although many of these associations dissipate over time, early-maturing girls are at risk of persistently higher delinquency and stronger negative peer influences. Copyright © 2014 by the American Academy of Pediatrics.