This study examined the stability of eating disorder (ED) classifications among a population-based sample of male and female adolescents (n = 2,516) who participated in Project EAT-II, a five-year longitudinal study. Cross-tabulations using weighted data identified diagnostic stability across six classifications (Anorexia Nervosa, Bulimia Nervosa, Binge Eating Disorder, Binge Eating and/or Compensatory Behaviors not meeting ED diagnosis, Body Image Disturbance without disordered eating, and Asymptomatic). One-third (32.6%) of adolescents who were asymptomatic at baseline and over half of those who were symptomatic at baseline reported symptoms five years later. All males and 82% of females with a threshold diagnosis at baseline remained symptomatic five years later, but rarely within the same classification. In conclusion, the presence of ED symptoms in adolescence strongly predicts ED symptoms five years later. ED diagnoses and classifications were unstable over time, underscoring the critical need for prevention efforts and periodic assessment and encouraging early detection and intervention among adolescents.