Objective: The present study aimed to examine whether Anorexia Nervosa (AN) illness severity or duration is associated with retention or treatment response in outpatient, enhanced cognitive–behavioral therapy (CBT-E). Method: Patients with a confirmed AN diagnosis (N = 134) completed measures of eating disorder symptoms and quality of life, and had their BMI objectively measured before, during, and after treatment. We evaluated whether illness severity or duration predicted treatment outcomes, using longitudinal regression models. Results: Greater levels of illness severity and duration were not associated with poorer treatment outcomes. Conclusions: Patients with more severe or long-standing AN illness did just as well in CBT-E as any other patient starting treatment. Therefore, classifying individuals as “severe and enduring” appears to lack clinical utility in CBT-E. Clinicians should continue to administer evidence-supported treatments such as CBT-E for patients with AN, regardless of duration or severity of AN illness.