Most Epstein-Barr virus (EBV) infections in infants and children are asymptomatic, while infection of adolescents or adults often results in infectious mononucleosis. The symptoms of infectious mononucleosis are primarily due to the T-cell proliferative response to EBV-infected B cells; thus, antiviral therapy does not affect the clinical course of disease. Failure of the cellular immune response to control EBV-induced B-cell proliferation can result in severe disease. Patients with the X-linked lymphoproliferative disease (XLPD) have a mutation in the SAP gene and EBV infection often leads to fatal infectious mononucleosis. Transplant recipients may develop lymphoproliferative disease, which often responds to treatment that enhances the immune response against EBV-infected B cells. About 50% of Hodgkin's and 20% of Burkitt's lymphomas in the United States contain EBV DNA. While chemotherapy and/or radiation therapy are mainstays of treatment, clinical trials are being developed using cytotoxic T cells directed against EBV proteins in these tumors.