PURPOSE: To estimate the overall annual incidence and age group distribution of eating disorders in a representative sample of adolescent female residents of Navarra, Spain. METHODS: We studied a representative sample of 2734 adolescent Navarran females between 13 and 22 years of age who were free of any eating disorder at the start of our study. Eighteen months into the study, we visited the established centers and the eating attitudes test (EAT-40) and eating disorder inventory (EDI) Questionnaires were administered to the entire study population. We obtained a final response of 92%. All adolescents whose EAT score was over 21 points and a randomized sample of those who scored 21 or below, were interviewed. Any person meeting the DSM-IV diagnostic criteria for Anorexia Nervosa (AN), Bulimia Nervosa (BN) or eating disorder not otherwise specified (EDNOS) was considered a case. RESULTS: We detected 90 new cases of eating disorders. Taking into consideration the randomly selected group whose EAT score was 21 points or below, we estimated the overall weighted incidence of eating disorders to be 4.8% (95% CI: 2.8-6.8), after 18 months of observation, in which EDNOS predominated with an incidence of 4.2% (95% CI: 2.0-6.3). The incidence of AN was 0.3% (95% CI: 0.2-0.5), while that of BN was also found to be 0.3% (95% CI: 0.2-0.5). The highest incidence was observed in the group of adolescents between 15 and 16 years of age. CONCLUSIONS: The overall incidence of ED in a cohort of 2509 adolescents after 18 months of follow-up was 4.8% (95% CI: 2.8-6.8), with EDNOS outweighing the other diagnoses. The majority of new cases of eating disorders were diagnosed between ages 15 and 16.