Objective: Use of disordered eating behaviors is common among adolescents, and cross-sectional research has suggested that friends may be an important influence, especially among females. The current study seeks to expand upon this literature using a longitudinal design and a large, diverse sample of male and female youth. Methods: A total of 2,516 adolescents provided survey data at baseline (1998-1999) and follow-up (2003-2004) regarding their friends' involvement in dieting and their own experience of chronic dieting, unhealthy weight control, extreme weight control, and binge eating. General linear modeling was used to generate predicted probabilities of disordered eating at follow-up across four levels of friends' dieting at baseline, adjusting for baseline use of disordered eating, and other covariates. Interaction terms were used to determine whether the association between friends' dieting and disordered eating differed across age cohorts. Results: One-third of participants reported that their friends were "not at all" involved in dieting at baseline, and 8.8% reported that their friends were very involved in dieting. Friends' dieting at baseline was positively associated with chronic dieting, unhealthy weight control behaviors, extreme weight control behaviors, and binge eating 5 years later among females, and with extreme weight control behaviors five years later among males. For both males and females, these associations were similar across age groups. Conclusions: Interventions targeting friendship groups rather than focusing solely on individuals may be an important strategy for the prevention of disordered eating. Health care providers may wish to ask adolescents about their friends' eating and dieting practices so as to address these issues in a clinical setting.