The development of B cell lymphoma has been reported to occur in recipients of a variety of organ transplants, including some patients who have received an allogeneic bone marrow graft. In this report, we describe a patient with severe aplastic anemia who developed a malignant B cell lymphoplasmacytoid proliferation 48 days after undergoing allogeneic marrow transplantation from her HLA-matched MLC-nonreactive brother. Immunologic studies showed this malignancy to be a mixed polyclonal and monoclonal proliferation in donor cells. Virologic studies documented Epstein Barr infection of the cells. A review of the literature suggests that graft-versus-host disease and treatment of this complication by antithymocyte globulin are related to the development of the EBV-related malignancy in the EBV-infected B cells of the developing bone marrow graft.