There is a growing interest in the evaluation of non-myeloablative conditioning therapy for allogeneic stem cell transplantation. Such regimens are expected to produce less toxicity while allowing both engraftment and a graft-versus-disease effect from the large number of donor-derived immunocompetent T lymphocytes given with the stem cells. Heavy immunosuppression used in recipients may have unexpected consequences. We describe the occurrence of a fatal Epstein-Barr virus-associated B cell lymphoproliferative disease (BLPD) early after such a non-myeloablated allogeneic peripheral blood stem cell transplant in a heavily pretreated patient.