T-bet, a T-box transcription factor, is expressed in CD4+ T lymphocytes committed to Th1 T-cell development and may participate in immunoglobulin class switching in B lymphocytes. T-bet is also expressed in a subset of T-cell lymphomas, particularly those that express other markers of Th1 T cell differentiation, and in a subset of B-cell non-Hodgkin's lymphomas. Because of the evidence that Hodgkin's lymphoma is a neoplasm of B cells, we examined cases of Hodgkin's lymphoma for T-bet expression by immunohistochemical staining and found that neoplastic cells in most cases of classic Hodgkin's lymphoma (33 of 37 cases, 89%), including nodular sclerosis type (17 of 21 cases, 81%) and mixed cellularity type (15 of 15, 100%), express T-bet. Neoplastic cells in most cases of nodular lymphocyte-predominant Hodgkin's lymphoma (15 of 18, 83%), a distinct clinical entity that differs from classic Hodgkin's lymphoma, also express T-bet. A Hodgkin's lymphoma-derived cell line, L1236, expresses T-bet by Western blot analysis as well as by immunohistochemical staining. In contrast, almost all cases of diffuse large B-cell lymphoma and most cases of anaplastic large cell lymphoma, neoplasms that may be confused with Hodgkin's lymphoma, are negative for T-bet. On that basis, T-bet should serve as a useful new marker for the diagnosis of Hodgkin's lymphoma. In addition, because T-bet expression is not detectable in the majority of reactive B cells, including germinal-center B cells, but is characteristically expressed by the neoplastic cells in Hodgkin's lymphoma, thought to be derived from germinal-center B cells, T-bet may play a role in Hodgkin's lymphoma oncogenesis.