This prospective study examined age of onset for binge eating and purging among girls during late adolescence and tested whether dieting and negative affectivity predicted these outcomes. Of initially asymptomatic adolescents, 5% reported onset of objective binge eating, 4% reported onset of subjective binge eating, and 4% reported onset of purging. Peak risk for onset of binge eating occurred at age 16, whereas peak risk for onset of purging occurred at age 18. Adolescents more often reported onset of a single symptom rather than multiple symptoms, and symptoms were episodic. Dieting and negative affectivity predicted onset of binge eating and purging. Findings suggest that late adolescence is a high-risk period for onset of bulimic behaviors and identify modifiable risk factors for these outcomes.