Congenital and acquired states of immunodeficiency are associated with an increased incidence of ill-defined lymphoproliferations. These are mainly B-cell, often extranodal, lymphoproliferations commonly associated with EBV. Their incidence is increasing with the rapid development of organ transplantation and spreading of the HIV infection. The association with EBV partly explains the pathogeny of these affections. This ubiquitous virus immortalizes B lymphocytes in vitro and is tumorigenic for new world primates. An EBV-specific cytotoxic T-cell memory prevents uncontrolled proliferation of infected B cells in normal subjects after the primary infection. The X-linked lymphoproliferative syndrome is a primary immunodeficiency characterized by an abnormal immune responsiveness to EBV, resulting in fatal infectious mononucleosis and malignant lymphoma. The severe immunosuppression present in transplanted patients allows EBV infected cells to proliferate, giving rise to a spectrum of lymphoproliferations. Reduction of immunosuppression alone is, in some cases, sufficient to produce tumor regression. The evolution of these affections is difficult to predict and requires a combined biological and clinical analysis, in order to evaluate the aggressivity of the tumor and the ability of the immune response of the host. HIV infected patients have an increased risk of developing Burkitt's lymphomas which are associated with EBV in 50% of the cases. Patients with AIDS at an advanced stage, present immunoblastic large cell lymphomas associated with EBV, similar to the post-transplant lymphomas. Lymphoproliferations in immunodeficiencies constitute a model for evaluation of the interaction between EBV and the immune system.