Epstein-Barr virus (EBV)–specific CD8 T lymphocytes are present at remarkably high frequencies in healthy EBV⁺individuals and provide protection from EBV-associated lymphoproliferative diseases. Allogeneic peripheral blood stem cell transplantation (allo-PBSCT) is a commonly used therapy in which T-cell surveillance for EBV is temporarily disrupted. Herein, human leukocyte antigen (HLA) class I tetramers were used to investigate the reestablishment of the EBV-specific CD8 T-cell repertoire in patients following allo-PBSCT. CD8⁺ T cells specific for lytic and latent cycle–derived EBV peptides rapidly repopulate the periphery of matched sibling allo-PBSCT patients. The relative frequencies of T cells specific for different EBV peptides in transplantation recipients closely reflect those of their respective donors. Investigation of patients at monthly intervals following unmanipulated allo-PBSCT demonstrated that the frequency of EBV-specific T cells correlates with the number of EBV genome copies in the peripheral blood and that expansion of EBV-specific T-cell populations occurs even in the setting of immunosuppressive therapy. In contrast, patients undergoing T-cell–depleted or unrelated cord blood transplantation have undetectable EBV-specific T cells, even in the presence of Epstein-Barr viremia. The protective shield provided by EBV-specific CD8 T cells is rapidly established following unmanipulated matched sibling allo-PBSCT and demonstrates that HLA class I tetramers complexed with viral peptides can provide direct and rapid assessment of pathogen-specific immunity in this and other vulnerable patient populations.