Isolated breast relapse after allogeneic hematopoietic stem cell transplantation (allo-HSCT) is less often seen. Chronic graft-versus-host disease (cGVHD) is effective in preventing marrow relapse, but cGVHD seems not to be effective extramedullary relapse (EMR). We report the case of isolated breast relapse after first allo-HSCT for acute lymphoblastic leukemia (ALL). A 47-year-old female was diagnosed with ALL achieved complete remission with salvage chemotherapy and underwent allo-HSCT from an HLA-matched sibling male donor. At 17 months post-transplant, she presented with a bilateral breast masses that confirmed the diagnosis lymphoblast involvement. She had no evidence of leukemia in her marrow that determined 100 % full-donor chimerism when she was relapsed in her both breasts.