We used an actor-partner interdependence model (APIM) to study the association between the individual group member and other group therapy members' defensive functioning on an individual group member's treatment outcome. We hypothesized that (a) more adaptive individual defensive functioning at pretreatment will be significantly related to better treatment outcomes (i.e., lower binge-eating and interpersonal distress) at 6 months post-treatment; and (b) more adaptive other group members' defensive functioning at pretreatment will be significantly related to better treatment outcomes at 6 months post-treatment. Participants (N = 136) were individuals with BED enrolled in group psychodynamic-interpersonal psychotherapy (GPIP). Participants completed attachment interviews and were assessed on interpersonal distress and days binged at pretreatment and 6 months post-treatment. The interview audio recordings were transcribed and used to code defensive functioning. We found that individual overall defensive functioning (ODF) scores at pretreatment were not significantly associated with binge-eating frequency or interpersonal distress at 6 months post-treatment. Other group members' mean ODF scores at pretreatment were significantly associated with individual interpersonal distress at 6 months post-treatment. However, the other group members' mean ODF scores were not significantly associated with individual binge-eating outcomes at 6 months post-treatment. Defensive functioning of other members of a therapy group may be particularly important for improving interpersonal functioning in individuals with BED. (PsycInfo Database Record (c) 2022 APA, all rights reserved).