Background: Anorexia Nervosa (AN) destroys developmentally important early years of many young people and knowledge is insufficient regarding course, treatment outcome and prognosis. Only a few naturalistic studies have been conducted within the field of eating disorder (ED) research. In this naturalistic study we included adolescents with AN or subthreshold AN treated in outpatient care, and the overall aim was to examine sample characteristics and treatment outcome. Additional aims were to examine potential factors associated with remission as an outcome variable, and possible differences between three time periods for treatment onset. Methods: Participants were identified through the Swedish national quality register for eating disorder treatment (SwEat), in which patients are registered at treatment onset and followed up once a year until end of treatment (EOT). Inclusion criteria were: medical or self-referral to one of the participating treatment units between 1999 and 2014, 13-19 years of age at initial entry into SwEat and diagnosed with AN or subthreshold AN. The total sample consisted of 3997 patient from 83 different treatment units. Results: The results show that 55% of the participants were in remission and approximately 85% were within a healthy weight range at EOT. Of those who ended treatment according to plan, 70% were in remission and 90% within a healthy weight range. The average treatment duration was approximately 15 months. About one third of the patients terminated treatment prematurely, which was associated with a decreased chance of achieving remission. Remission rates and weight recovery increased over time, while treatment duration decreased. Considering treatment outcome, the results did not show any differences between patients with AN or subthreshold AN. Conclusions: The present study shows a relatively good prognosis for adolescent patients with AN or subthreshold AN in routine care and the results indicate that treatment for adolescents with ED in Sweden has become more effective over the past 15 years. The results of the present study contribute to the scope of treatment research and the large-scale naturalistic setting secures the generalizability to a clinical environment. However, more research is needed into different forms of evidence, new research strategies and diversity of treatment approaches.