Similarities have been noted in the histologic patterns of lymphomatoid granulomatosis and Epstein-Barr virus associated lymphoproliferative disease involving the lung. Epstein-Barr virus has also been identified by polymerase chain reaction in most cases of lymphomatoid granulomatosis; however, the precise cellular localization of Epstein-Barr virus sequences has not been extensively studied. We analyzed 10 cases of lymphomatoid granulomatosis involving the lung by immunohistochemistry and combined immunohistochemistry with in situ hybridization for Epstein-Barr virus, CD20, and CD45RO. All cases were selected from the files of the Armed Forces Institute of Pathology and met the clinical and histologic criteria for the diagnosis of lymphomatoid granulomatosis, grades 1 through 3. In all 10 cases, immunohistochemistry showed that most of the cells--small to medium-sized lymphocytes--were T cells (CD45RO+); however, a much smaller population of medium-sized to large atypical cells were B cells (CD20+). In each case, combined immunohistochemistry and in situ hybridization confirmed the presence of Epstein-Barr virus sequences within B (CD20+) cells and the absence of Epstein-Barr within T-cells (CD45RO+). Polymerase chain reaction analysis for immunoglobulin heavy-chain gene rearrangement identified a monoclonal pattern in six of nine cases tested, whereas analysis for T-cell receptor gamma-chain gene rearrangements was negative in three cases tested. On the basis of these findings, we hypothesize that most cases of lymphomatoid granulomatosis involving the lung represent a proliferation of Epstein-Barr virus infected B-cells with a prominent T-cell reaction and vasculitis, distinguishing these cases from angiocentric "T-cell lymphomas" in other sites, such as the head and neck.