The effectiveness of group psychotherapy has been empirically studied and supported over several decades; however, there remains much to understand regarding the specific factors contributing to effective group psychotherapy. The current study uses Kashy and Kenny's (2000) actor-partner interdependence model (APIM) to examine the relationship between an individual group member's outcome and the outcomes of the other group members. This is the first study to examine the effects of the outcomes of other therapy group members on those of individual members. Specifically, we examined the relationship between an individual group member's presymptom score, the aggregated presymptom scores of the other group members, and the aggregated pre- to postsymptom change of the other group members on an individual group member's pre- to postsymptom change. We analyzed the change in pre-post posttraumatic stress disorder (PTSD) symptoms of 105 women in 16 trauma-informed groups in a women's correctional facility. As hypothesized, an individual's presymptom measure (actor effect) and the aggregated presymptom measures of the other group members (partner effect) were positively related to the individual's change in PTSD symptoms. Contrary to our hypothesis, the aggregated pre-post change in PTSD symptom measures of the other group members (partner effect) was negatively associated with the pre-post change in PTSD symptom measures of an individual group member. Social comparison theory is discussed as an explanation for why a group member would report lowered amounts of change when in a group with others who are reporting a higher amount of change. Implications for research and practice are discussed. (Contains 1 table.)