The bcl-3 gene at chromosome 19q13 encodes a member of the lkappaB family involved in regulating the NFkappaB pathway. Originally identified by its involvement in the rare t(14:19)(q32;q13), BCL-3 expression has never been analyzed in a wide variety of lymphomas. We assessed BCL-3 expression in 353 cases of non-Hodgkin lymphoma and Hodgkin lymphoma using formalin-fixed, paraffin-embedded tissue specimens, a monoclonal antibody specific for BCL-3, and immunohistochemical methods. Of 172 B-cell lymphomas, 10 (6%) were positive for BCL-3, including six of 23 (26%) diffuse large B-cell lymphoma, one of 17 (6%) small lymphocytic lymphoma, one of 26 (40%) follicular lymphoma, and two of 49 (4%) mantle cell lymphoma. All other B-cell neoplasms were negative, including marginal zone lymphoma (n = 24, 11 extranodal, nine nodal, four splenic), Burkitt lymphoma (n=10), lymphoplasmacytic lymphoma (n=10), lymphoblastic lymphoma (n=8), and plasmacytoma (n=5). Of 111 T/NK-cell lymphomas, 25 (23%) were positive for BCL-3, including 13 of 40 (32%) anaplastic large-cell lymphoma, three of 10 (30%) angioimmunoblastic T-cell lymphoma, two of eight (25%) extranodal NK/T-cell lymphoma of nasal type, three of 12 (25%) mycosis fungoides, one of five (20%) enteropathy-type T-cell lymphoma, and two of 21 (10%) peripheral T-cell lymphoma unspecified. All other T-cell neoplasms were negative, including lymphoblastic lymphoma (n=6), prolymphocytic leukemia (n=6), and subcutaneous panniculitis-like T-cell lymphoma (n = 3). Of 70 Hodgkin lymphomas, of all types, 29 (41%) were positive for BCL-3. The relatively high frequency of BCL-3 expression in some non-Hodgkin and Hodgkin lymphoma types raises the possibility that BCL-3 is involved in the pathogenesis of these tumors, and may be a target of new therapies.