Fifteen patients with chemosensitive metastatic breast cancer (MBC) underwent reduced intensity (RIST) allo-SCT between 1999 and 2006. The purpose of this single-center study was to evaluate the feasibility, safety and efficacy of this therapeutic approach. The pretransplant conditioning regimen consisted of fludarabine (25 mg/m(2) at days -5 to -1) and CY (60 mg/kg at days -2, -1). Stem cells were from HLA-matched sibling donors. The treatment-related mortality was 2/15 (13%). Median PFS and OS were 144 days (43-509 days) and 303 days (122-1376 days), respectively. The 1-year PFS was 20%, and the 1-year and 2-year OS was 40 and 20%, respectively. No objective tumor responses were observed, but the relatively long PFS does suggest a graft-vs-tumor effect. Although RIST using this CY/fludarabine regimen is feasible, the efficacy in this set of patients was limited. Future clinical trials should be performed to improve the knowledge of mechanisms of antitumor effects in breast cancer.