The application responds to PAR-21-130: Clinical Trials to Test the Effectiveness of Treatment, Prevention, and Services Interventions. With an incidence rate of about 1%, Anorexia Nervosa (AN) is a serious mental disorder associated with high mortality, morbidity and cost but becomes highly resistant once it has taken an enduring course. The first-line treatment for adolescents with AN is Family Based Treatment (FBT), but is not available to many patients. An effective strategy used with other eating disorders to address impediments to scaling effective treatments is guided self-help (GSH) versions of efficacious treatments. We developed, refined, and tested GSH-FBT in case series and pilot studies. Results of a multisite randomized feasibility study comparing GSH-FBT to FBT-V found that both treatments were similarly acceptable to families with good recruitment rates, low attrition rates, high completion rates of assessments, and were safe to deliver while achieving similar improvements in eating related cognitions and weight. However, GSH-FBT achieved these outcomes with greater efficiency (e.g., larger ratio of improvement in weight and cognitions to therapist treatment time) by utilizing approximately 75% less therapist time than FBT-V. The main aim of this proposed comparative effectiveness study is to confirm that clinical improvements in GSH-FBT are achieved with greater efficiency than FBT-V in generalizable clinical settings. If this outcome is confirmed, it will lead to an increased opportunity to utilize and scale an effective treatment for adolescent AN, promote increased access and improve outcomes for these patients. We will also explore therapist attitudes toward GSH-FBT. To conduct this study 200 adolescents with DSM-5 AN and their families will be randomized to either GSH-FBT or FBT-V in two treatment sites (Stanford Children’s Hospital Eating Disorder Clinic and the Ontario Provincial Network of Ministry-funded Specialized Treatment Services for Eating Disorders). The main outcome will be clinical efficiency (the ratio of change in weight and eating related distorted cognitions to therapist time). Parental self- efficacy will be assessed as a potential mediator of treatment effect. Family structure and severity of eating related obsessions will be examined as moderators. Weights will be collected from session 1-4 to assess early weight gain as a predictor of weight remission at the end of treatment (EOT). Data on therapists’ views of GSH-FBT implementation will be collected by quantitative measures at BL and EOT, as well as by individual semi-structured qualitative interviews. Both primary and secondary outcomes will be analyzed in line with the intention to treat principle.