PROJECT SUMMARY/ABSTRACT Anorexia nervosa (AN) is a costly and life-threatening illness. Patients with severe AN are often treated in the acute setting, but relapse after treatment is common. Further, the discharge criterion in acute settings is typically weight restoration. However, research has indicated that weight-based recovery is not “enough” and that full recovery involves physical, behavioral, and psychological components. Cognitive-behavioral therapy (CBT) is useful with this population, including following discharge, and may aid in addressing psychological symptoms, achieving full recovery, and decreasing relapse. However, very few patients have access to providers trained in these techniques. mHealth technologies have great potential to exponentially increase access to high-quality services for the post-acute treatment of AN by addressing barriers to treatment, but to date, research on these approaches has been extremely limited. Our team has successfully developed a guided self-help CBT-based mobile app, StudentBodies-Eating Disorders (SB-ED), which has demonstrated effectiveness among college women with binge/purge-type EDs. At the same time, we have demonstrated support for in-person CBT for the post-acute care and relapse prevention of adult AN. The overarching goal of this R34 is to develop and test a CBT-based, coached mobile app to address the post-acute care and relapse prevention of AN, based on: 1) our own app for the treatment of binge/purge-type EDs; 2) our work on in- person use of CBT for post-acute care of AN; and 3) research on an additional feature that has high theoretical potential for supporting individuals with AN in the post-acute period—a social networking feature that provides the opportunity for positive interactions with individuals with shared experiences. In Aim 1 of the proposed work, guided by an Advisory Board of relevant stakeholders, we will adapt the existing SB-ED app for this population and add a social networking feature. Then, we will further refine the intervention through three rounds of usability testing with N=20 total. In Aim 2, we will conduct a pilot randomized controlled trial (N=90) of the mobile intervention versus the mobile intervention plus social networking versus treatment as usual (TAU). We will estimate the preliminary feasibility and effectiveness of the mobile intervention versus the mobile intervention plus social networking versus TAU in terms of reducing ED psychopathology (primary outcome), reducing ED behaviors, achieving weight maintenance, reducing depression and suicidal ideation, and reducing clinical impairment. We will also examine whether the mobile intervention changes the proposed targets and whether changes in targets are associated with clinical benefit, as well as conduct exploratory analyses to identify within-app predictors and moderators of outcome. Effect size and attrition estimates will aid in the planning of an R01 randomized controlled trial where we will...