Evaluation of a Twenty-session Cognitive-behavioral Therapy With Anorexia Nervosa (CBT-20-AN) Among Adults (18+)

The study is a clinical trial assessing the efficacy of a new twenty session cognitive behavioral therapy for adults (18+) with anorexia nervosa (AN). It is expected that participants will gain a significant amount of weight and experience a significant decrease in eating disorder psychopathology and behavior from pre- to post- treatment and that this will be retained after 6-months.

Inclusion Criteria: 1. Eligible patients must meet DSM-5 criteria for anorexia nervosa. This diagnosis can be conferred via the EDE 17.0D. 2. Age \> 18 3. Patients must be able to provide informed consent and comply with the study procedures. 4. Patients must be able to complete computer-based self-report questionnaires with adequate accommodation, if necessary. 5. Patients must have a primary care physician, Adolescent Medicine physician, or consulting physician with eating disorder expertise at Massachusetts General Hospital/Massachusetts General Brigham network who can provide ongoing medical monitoring while the patient takes part in the study. Exclusion Criteria: Patients will be excluded if they exhibit: 1. BMI \< 15 or \> 19, obtained via patient medical chart or from study 2023P000469 or 2018P002915. 2. HR \100, obtained via patient medical chart or from study 2023P000469 or 2018P002915. 3. Systolic BP (SBP and DBP) \160, obtained via patient medical chart or from study 2023P000469 or 2018P002915. 4. Temperature \< 96.5 F, obtained via patient medical chart or from study 2023P000469 or 2018P002915. 5. Not eating food by mouth and/or any tube feeding, per EDE 6. Purging \> 4x/week, per EDE. 7. Active suicidality (suicidal ideation with intent or plan) to the point that more intensive treatment (i.e. acute hospitalization) is required 8. Active untreated and unstable bipolar disorder (i.e. stable bipolar disorder under care of a psychiatrist is allowed) 9. Psychosis 10. Intellectual disability 11. Any condition that, after the baseline evaluation, is determined to preclude treatment with CBT. 12. Concurrent psychotherapy. Patients must be willing to take a pause from any ongoing psychotherapy in order to focus on CBT-AN-20, which is in alignment with standard clinical care. 13. Symptoms that are not appropriate for outpatient level of care and therefore require a higher level of care (i.e. BMI \