BACKGROUND: Dieting and unhealthy weight control behaviors are common among adolescents and questions exist regarding their long-term effect on weight status. OBJECTIVE: To examine 10-year longitudinal associations between dieting and unhealthy weight control behaviors and changes in body mass index (BMI) from adolescence to young adulthood. METHODS AND PROCEDURES: A diverse population-based sample of middle school and high school adolescents participating in Project EAT (Eating and Activity in Teens and Young Adults) was followed up for 10 years. Participants (N = 1,902) completed surveys in 1998-1999 (Project EAT-I), 2003-2004 (Project EAT-II), and 2008-2009 (Project EAT-III). Dieting and unhealthy weight control behaviors at Time 1 and Time 2 were used to predict 10-year changes in BMI at Time 3, adjusting for sociodemographic characteristics and Time 1 BMI. RESULTS: Dieting and unhealthy weight control behaviors at both Time 1 and Time 2 predicted greater BMI increases at Time 3 in males and females, as compared with no use of these behaviors. For example, females using unhealthy weight control behaviors at both Time 1 and Time 2 increased their BMI by 4.63 units as compared with 2.29 units in females not using these behaviors (p < .001). Associations were found in both overweight and nonoverweight respondents. Specific weight control behaviors at Time 1 that predicted larger BMI increases at Time 3 included skipping meals and reporting eating very little (females and males), use of food substitutes (males), and use of diet pills (females). CONCLUSIONS: Findings clearly indicate that dieting and unhealthy weight control behaviors, as reported by adolescents, predict significant weight gain over time.