OBJECTIVE: To identify the socio-environmental, personal and behavioural factors that are longitudinally predictive of changes in adolescents' fast-food intake. DESIGN: Population-based longitudinal cohort study. SETTING: Participants from Minnesota schools completed in-class assessments in 1999 (Time 1) while in middle school and mailed surveys in 2004 (Time 2) while in high school. SUBJECTS: A racially, ethnically and socio-economically diverse sample of adolescents (n 806). RESULTS: Availability of unhealthy food at home, being born in the USA and preferring the taste of unhealthy foods were predictive of higher fast-food intake after 5 years among both males and females. Among females, personal and behavioural factors, including concern about weight and use of healthy weight-control techniques, were protective against increased fast-food intake. Among males, socio-environmental factors, including maternal and friends' concern for eating healthy food and maternal encouragement to eat healthy food, were predictive of lower fast-food intake. Sports team participation was a strong risk factor for increased fast-food intake among males. CONCLUSIONS: Our findings suggest that addressing socio-environmental factors such as acculturation and home food availability may help reduce fast-food intake among adolescents. Additionally, gender-specific intervention strategies, including working with boys' sports teams, family members and the peer group, and for girls, emphasizing the importance of healthy weight-maintenance strategies and the addition of flavourful and healthy food options to their diet, may help reduce fast-food intake.