Adoptive transfer of antigen-specific cytotoxic T lymphocytes (CTLs) offers safe and effective therapy for certain viral infections and could prove useful in the eradication of tumor cells. Whether or not the infused T cells persist for extended periods, retaining their ability to expand in response to antigenic stimulation, is not known. We now report long-term detection of gene-marked Epstein-Barr virus (EBV)-specific CTLs in immunocompromised patients at risk for the development of EBV lymphoproliferative disease. Infusions of CTLs not only restored cellular immune responses against EBV, but also established populations of CTL precursors that could respond to in vivo or ex vivo challenge with the virus for as long as 18 months. Our findings support wider use of antigen-specific CTLs in adoptive immunotherapy.