The Epstein-Barr virus (EBV) has been shown to be associated with posttransplant lymphoma, Hodgkin's disease, and T-cell lymphoma, in addition to African Burkitt's lymphoma. In a retrospective study of 56 consecutive cases of T-cell lymphoma, EBV DNA was found by Southern blot and in situ DNA hybridization in 10 (20%) of 50 peripheral T-cell lymphomas, but in none of six cases of T-lymphoblastic lymphoma. Peripheral T-cell lymphomas containing EBV DNA could be subclassified into three categories according to histology and immunophenotypic studies: (1) T-cell lymphoma of the helper phenotype, five cases. Two cases had histologic features resembling angioimmunoblastic lymphadenopathy (AILD). (2) T-cell lymphoma of the cytotoxic/suppressor phenotype, four cases. AILD-like features could also be recognized in two cases. Reed-Sternberg-like giant cells were identified in three cases designated Hodgkin-like T-cell lymphoma. (3) Angiocentric T-cell lymphoma or lymphomatoid granulomatosis in one case, initially affecting the skin and nose; no T-cell subset could be defined. Six of the eight EBV DNA-positive patients tested for serum EBV antibodies had elevated titers of IgG antiviral capsid antigen (greater than 640) and/or early antigen (greater than 10). From combined studies of Southern blot hybridization by using EBV termini fragment probe and in situ DNA hybridization, the EBV genomes appeared to be clonotypically proliferated in the neoplastic T cells. The patients in all three groups usually had prolonged fever preceding the diagnosis, hepatosplenomegaly, an aggressive clinical course, and poor response to chemotherapy; nine died with a median survival of only 8 months. We propose that these EBV-associated aggressive T-cell lymphomas, like human T-cell leukemia/lymphoma virus-positive T-cell lymphoma, have characteristic clinicopathologic features and should be treated as a separate disease entity.