A 20-year-old woman was hospitalized on November 11, 1994 with Behçet's disease-like symptoms (fever, genital ulcer and aphtha in the oral cavity). Bilateral cervical lymph node swelling was also noted and diagnosed as lymphadenitis on biopsy. Chronic active Epstein-Barr virus infection (CAEBV) was diagnosed based on the high titer of antibodies to the EBV capsid antigen, early antigen, and nuclear antigen. She was treated with prednisolone and acyclovir and all symptoms improved. However, ten months after onset of symptoms, T-cell malignancy was diagnosed on bone marrow aspiration, which revealed 34.9% blast cells that had rearrangement of TCR-beta. She died on May 8, 1995, despite anticancer therapy. In analyzing the blast cells, the monoclonal junctional DNA structure of the EBV terminal repeat was analyzed by Southern blotting and provided definitive evidence for the monoclonality of EBV-infected T cells. These findings strongly suggest that EBV plays a pathogenic role in T-cell malignancy. EBV-infected T-cell malignancy, such as this case, is very rare in Japan, especially in adult.