Purpose: To evaluate the effect of a standardized group psychosocial intervention on health-related quality of life (HrQOL) in women with metastatic breast cancer and to explore the effect of missing data in HrQOL analyses. Patients and Methods: Between 1993 and 1998, seven Canadian centers randomly assigned 235 eligible women to participate in a weekly, 90-minute, therapist-led support group that adhered to principles of supportive-expressive (SE) therapy or to a control arm (no SE). All women received educational material and any type of medical or psychosocial care deemed necessary. HrQOL data were prospectively collected using the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire C30 (EORTC QLQ-C30) at baseline, 4, 8, and 12 months. The primary HrQOL analyses compared scores in the two study arms. Analyses were limited to women with appropriate baseline HrQOL information (n = 215). Results: Baseline EORTC QLQ-C30 scores were not different between the two study arms (all P > .05). Primary analysis of all subscales failed to show a significant influence of the intervention on HrQOL (all P > .05). There was a significant deterioration over time in several functional scales of the EORTC QLQ-C30: global (P = .03), physical (P = .0002), role (P = .01), and cognitive functioning (P = .04); and in symptom scales: dyspnea (P = .007), appetite loss (P = .04), and fatigue (P = .003); these changes were independent of randomization allocation. Results were similar in additional analyses of overall HrQOL using a variety of approaches to handling missing data. Conclusion: Supportive-expressive group therapy in patients with metastatic breast cancer does not appear to influence HrQOL, as measured by the EORTC QLQ-C30.