We reviewed 66 women with poor-risk metastatic breast cancer from 15 centers to describe the efficacy of allogeneic hematopoietic cell transplantation (HCT). Median follow-up for survivors was 40 months (range, 3-64). A total of 39 patients (59%) received myeloablative and 27 (41%) reduced-intensity conditioning (RIC) regimens. More patients in the RIC group had poor pretransplant performance status (63 vs 26%, P=0.002). RIC group developed less chronic GVHD (8 vs 36% at 1 year, P=0.003). Treatment-related mortality rates were lower with RIC (7 vs 29% at 100 days, P=0.03). A total of 9 of 33 patients (27%) who underwent immune manipulation for persistent or progressive disease had disease control, suggesting a graft-vs-tumor (GVT) effect. Progression-free survival (PFS) at 1 year was 23% with myeloablative conditioning and 8% with RIC (P=0.09). Women who developed acute GVHD after an RIC regimen had lower risks of relapse or progression than those who did not (relative risk, 3.05: P=0.03), consistent with a GVT effect, but this did not affect PFS. These findings support the need for preclinical and clinical studies that facilitate targeted adoptive immunotherapy for breast cancer to explore the benefit of a GVT effect in breast cancer.