The benefits of allografting noted in some malignant diseases might be safely extended to metastatic breast cancer by a combination of cytoreduction with high-dose chemotherapy (HDT) and autologous stem-cell transplant (ASCT) with graft-versus-tumour effect mediated by transplanted donor immune cells with nonmyeloablative allografting (reduced intensity conditioning transplantation, RICT). 17 patients with heavily pretreated disease were given tandem transplants. 13 patients sustained donor engraftment. Three had partial remission after HDT and ASCT and complete remission after RICT; they achieved full chimerism and all developed graft-versus-host disease (GVHD) before regression of cancer. Another patient did not respond to HDT and ASCT but had partial remission after RICT, giving an overall response rate of 24%. Five patients had grade II or higher acute GVHD and five had extensive chronic GVHD. No non-relapse-related deaths occurred during the first 100 days. Five patients (29%) were alive 90-2160 days (median 1320) after RICT. This two-step approach is feasible in patients with metastatic breast cancer.