Evidence supporting outpatient treatments for anorexia nervosa (AN) is severely lacking, due to low retention and poor outcome. One explanation for drop-out is weak treatment alliances. A single-case experimental analysis accompanied by in-depth qualitative description is presented for Ms. O, who received a novel treatment for AN called Alliance Focused Treatment (AFT) that attends to ruptures in the alliance, interpersonal difficulties and emotional avoidance. At intake Ms. O met diagnostic criteria for AN, Major Depressive Disorder, and Social Phobia. She was characterized as having symptoms of Obsessive Compulsive, Avoidant, and Depressive personality disorders. Treatment began with a Baseline followed by the experimental (AFT) and comparison treatments (Behavioral Change Treatment [BCT]) using a replicated experimental single-case phase change design. Graphs of slopes of kilocalorie and alliance change facilitated observation of treatment effects. Ms. O participated in 16 sessions of AFT and 8 sessions of BCT with specific benefits. Ratings of the treatment alliance were consistently high and she evidenced significant changes in weight, quality of life, and personality pathology. Associations between rupture/repair episodes and kilocalorie increases were observed. The utility of the treatment relationship in facilitating emotional expression was evident. At posttreatment, Ms. O endorsed cognitive AN symptoms, although these were not explicitly treated. This study provides preliminary support for the feasibility and effect of AFT and BCT, and highlights the importance of the alliance in treating adults with AN. Further research on emotion regulation in AN and its effect on the treatment relationship are needed.