We describe the development of Epstein-Barr-virus (EBV)-related lymphoproliferative disease (LPD) in the recipient of a histocompatible bone marrow transplant (BMT). Although this rare complication is more common in recipients of mismatched bone marrow, several distinguishing features of our case may have contributed to the development of LPD in the recipient of a matched bone marrow transplant. The patient had received marrow from a sibling with Trisomy 21, a syndrome associated with variable cellular and humoral immune defects. Our patient also was infected with cytomegalovirus and was treated with immunosuppressant therapy for graft versus host disease. Although development of LPD in transplant recipients is a multifactorial process, either acquired or congenital immunosuppression/dysregulation is a common prerequisite for the process. Our case suggests that subtle immune defects in individuals with Down syndrome may contribute to the immunosuppressed setting in which EBV-related LPD can develop.