Epstein-Barr virus (EBV)—associated lymphoproliferative disorder (LPD) is a serious complication after allogeneic bone marrow transplantation (BMT). Dynamics of EBV-specific cytotoxic T lymphocytes (CTL), which are important in controlling EBV during the LPD, have not been fully elucidated. A patient with Wiskot-Aldrich's syndrome was diagnosed as suffering from LPD on day 47 after BMT. Fluorescence-activated cell sorter (FACS) analysis for interferon-γ production revealed that >70% of the patient's CD8⁺ T cells were EBV specific. The patient's lymphocytes were directly cytotoxic to donor-derived EBV-positive lymphoblastoid cells, which was blocked by an anti—class I antibody. EBV-specific CD8⁺ T cell counts declined in parallel with EBV genome load, and full recovery of LPD was obtained with relaxation of immunosuppressive drugs. The results illustrate longitudinal dynamics of EBV-specific CTL during the posttransplant LPD; they also illustrate the advantages of using FACS analysis for EBV-specific CTL to make decisions about treatment.