T cells expressing the gamma delta T cell receptor have been implicated in anti-Epstein-Barr virus (EBV) immunity and in the pathogenesis of autoimmune diseases of the central nervous system. However, they have never been isolated from human brain tissue for direct analysis. We now report a 6-year-old girl with EBV-associated posttransplantation lymphoproliferative disease involving inflammatory brain lesions. A high proportion of gamma delta T cells was found in the blood and in a brain lesion. Cultured T cell lines were found to have a remarkably high frequency of responsiveness to an EBV-transformed line, with a 17-fold enrichment in the brain lesion. These T cells expressed predominantly the gamma delta T cell receptor and mediated non-MHC-restricted cytotoxicity against EBV-infected target cells. These results provide the first demonstration of an association of gamma delta T cells with a posttransplantation lymphoproliferative disease and suggest a role of gamma delta T cells in mediating inflammatory processes in the brain.