The association of Epstein-Barr virus (EBV) with human immunodeficiency virus-negative T-cell lymphoma was examined in 68 patients using the polymerase chain reaction (PCR) with DNA obtained from formalin-fixed paraffin-embedded tissues and an in situ hybridization technique. EBV-encoded RNA (EBER) was detected in 43 of 68 cases (63%) of peripheral T-cell lymphoma: in 100% (11 of 11 cases) of NK/T-cell lymphomas, 70% (14 of 20 cases) of angioimmunoblastic T-cell lymphomas (AILT) and 49% (18 of 37 cases) of other types of peripheral T-cell lymphoma. A positive band was also detected at high incidence (36 of 65 cases; 55%) in a PCR analysis using primers to detect the Bam HI-W fragment of EBV. In the immunohistochemical analysis using a monoclonal antibody to latent membrane protein 1 (LMP-1) of EBV, one of the EBV-encoded latent gene products, LMP-1, was found to be expressed in 13 of 64 cases (20%), but EBNA-2 was not expressed in all the cases examined (0 of 59 cases; 0%). The 5-yr survival rate was 28% for peripheral T-cell lymphomas overall, 0% for NK/T-cell lymphomas, 38% for AILTs and 28% for other types of peripheral T-cell lymphoma. The difference in the overall survival rate between NK/T-cell lymphoma and non-NK/T-cell lymphoma was significant (P = 0.0498 by Log-rank test). Among peripheral T-cell lymphoma patients overall, the group severely infected with EBV (EBER-ISH ++) had a lower 5-yr survival rate (8%) than the group slightly (EBER-ISH +) or not infected (38%; P = 0.0013).