Post-transplant lymphoproliferative disorder (PTLD) is a rare but life-threatening complication after solid organ and haematopoietic stem cell transplantation. A 40-year-old woman who was diagnosed as having refractory anaemia with ring sideroblast 6 years ago took an ABO mismatched, unrelated allogeneic haematopoietic stem cell transplantation (HSCT) from a 32-year-old healthy male donor. The bone marrow (BM) study was carried out because of progressing pancytopenia, serum biclonal gammopathy and a distorted ratio of serum level of free κ and λ light chain 138 days after HSCT. The BM examination showed an increased number of plasma cells (12% of total marrow cells) comprising mainly CD45-CD19-CD138+ malignant plasma cells with an immunoglobulin heavy-chain gene rearrangement. Conventional cytogenetics and molecular personal identification studies revealed that all BM cells were totally replaced by donor cells, thus indicating the donor cell origin of PTLD-multiple myeloma. The BM microenvironment of the recipient might be associated with the development of PTLD-multiple myeloma.