Posttransplant lymphoproliferative disorder (PTLD) is a well recognized, although relatively uncommon, complication of both solid organ and allogeneic bone marrow transplantation. In most cases, PTLD is associated with Epstein-Barr virus (EBV) infection of B cells, either as a consequence of reactivation of the virus posttransplantation or from primary posttransplantation EBV infection acquired from the donor. While T-cell lymphoproliferative disorders not associated with EBV infection have also been documented after solid organ and bone marrow transplantation, the vast majority are B-cell proliferations.