Within a multisite study, including 289 inpatients from six different hospitals who underwent interpersonal-psychodynamic group psychotherapy, associations among attachment characteristics, therapeutic factors, and treatment outcome were investigated. Attachment characteristics were assessed with an interview-based measure (Adult Attachment Prototype Rating [AAPR]) as well as an attachment self-report (Bielefeld Questionnaire of Client Expectations [BQCE]). Therapeutic factors were measured retrospectively with the Dusseldorf Therapeutic Factors Questionnaire and treated as an individual- as well as a hospital-specific characteristic. On an individual level, only the group climate factor independently predicted treatment outcome (i.e., Symptom Checklist-90-R Global Severity Index and Inventory of Interpersonal Problems mean). If simultaneously but separately included into a path model, analyses revealed independent significant effects of AAPR-Security and BQCE-Security on group climate. If modeled as a latent variable (common attachment security), a substantially higher proportion of group climate variance could be explained. Further analyses revealed interactions between particular therapeutic factors and attachment characteristics, indicating a particular importance of these therapeutic factors for different attachment categories.