X-linked lymphoproliferative disease (XLP) is a rare genetic syndrome that continues to serve as a useful model to understand more broadly the role of immunodeficiency and the pathogenetic mechanisms for the spectrum of Epstein-Barr virus (EBV)-induced diseases to which XLP is predisposed. Apart from XLP, EBV infection is related to the high frequency of non-Hodgkin's lymphoma in children with various primary immune deficiency diseases and in allograft recipients. More recently, EBV has been implicated in several lymphoproliferative diseases in individuals with acquired immune deficiency syndrome. Studies thus far on patients with XLP suggest that immune deficiency is a major determinant of these diseases. Additional molecular aberrations must be necessary in the pathogenesis of lymphoma to convert polyclonal to monoclonal disease.