PURPOSE: Frequent fast food intake is associated with poorer diet quality and greater weight gain. The aims of this study were to describe changes in fast food intake during the transition from middle adolescence to young adulthood, and to identify baseline correlates of this eating behavior in early young adulthood. METHODS: Data were drawn from Project EAT, a population-based, longitudinal study in Minnesota. Surveys were completed by 935 females and 751 males in high school classrooms at baseline (1998-1999; mean age = 15.9 years) and by mail at follow-up (2003-2004; mean age = 20.5 years). RESULTS: Frequent intake of fast food (> or =3 times/week) was reported by 24% of males and 21% of females during adolescence. At follow-up, in early young adulthood the eating behavior increased among males (33%, p < .001), and there was no further increase among females (23%; p = .16). Baseline snack frequency was positively associated with frequency of fast food intake at follow-up among both genders. Baseline peer support for healthy eating among males and both concern about health and self-efficacy for healthy eating among females were inversely related to follow-up fast food intake. Among females, baseline perceptions of time and taste barriers to healthy eating, lunch frequency, television viewing, and unhealthy food availability at home were also positively associated with follow-up fast food intake. CONCLUSIONS: Interventions are needed to address the high prevalence of frequent fast food intake among adolescents and young adults. Health professionals should help young people identify convenient and healthful food choices for meals and snacks consumed away from home.