Lymphoproliferative disorders involving uncontrolled expansion of donor-derived B cells infected with Epstein-Barr virus (EBV) are a significant problem after hemopoietic stem cell transplantation. Risk factors, which include T cell depletion, major histocompatibility complex mismatch, and intensity of immunosuppression illustrate the importance of T cell immune surveillance. Recent studies have identified viral and host factors that affect the T-cell response to EBV. Monitoring EBV load in the blood by polymerase chain reaction allows early identification of high-risk patients and early institution of therapy. Adoptive immunotherapy approaches using donor T cells have proven effective and EBV-specific cytotoxic T lymphocytes have also been used successfully for prophylaxis. The simplest way of preventing EBV lymphoproliferation, however, may be to deplete B cells from the donor marrow prior to infusion to prevent the transmission of EBV-infected B cells.