Epstein-Barr virus (EBV) is a ubiquitous gammaherpesvirus that is associated with a variety of malignancies. In vivo infection of B lymphocytes is initially associated with the broad expression of immunodominant viral latency genes and proliferation of infected cells. Ultimately, a viral reservoir is established in resting B cells with restricted expression of viral latency genes and no expression of immunodominant viral genes. Among the tumours associated with EBV that are relevant to a consideration of EBV in HIV-associated malignancies are posttransplant lymphoproliferative disease, Burkitt's lymphoma (BL) and Hodgkin's disease (HD). BL carries whereas EBV in only a minority of cases whereas HD in patients infected with HIV is virtually always EBV-associated. EBV-directed T cell therapies have proven effective in posttransplant lymphomas in bone marrow transplantation patients. In patients with HIV infection, primary central nervous system (CNS) and immunoblastic lymphomas show similarities with posttransplant lymphoproliferative disease. EBV detection studies in cerebrospinal fluid are useful diagnostically in primary CNS lymphoma. T cell therapies may be useful in the treatment of EBV-associated lymphomas. Thus, a better understanding of the relationship between EBV and these tumours will not only help to clarify their pathogenesis, but may facilitate the development of new diagnostic and therapeutic strategies.