Chronic active Epstein-Barr virus (EBV) infection is manifested clinically by the persistence of infectious mononucleosis-like symptoms or its complications for a prolonged period ranging from one to several years. This syndrome may include severe disease manifestations and can be fatal. The role of EBV in the pathogenesis of chronic active EBV infection has been unclear. We investigated two Japanese patients with severe chronic active EBV infection who subsequently developed EBV-positive T-cell lymphoma. We found that the patients had evidence of EBV infection in the peripheral blood CD4+ T-cells 19 and 3 months, respectively, before the T-cell lymphoma was diagnosed. The lymphomas were infected with monoclonal EBV and expressed the EBV latency genes EBNA-1, LMP-1, and LMP-2A, a virus latency pattern referred to as latency II. Genetic studies showed that the virus detected in the T-cell lymphoma was indistinguishable from the virus in the peripheral blood CD4+ T-cells. These studies support an important pathogenetic role of T-cell infection with EBV in chronic active EBV infection and in the EBV-positive T-cell lymphoma that followed.