Immunodeficiency-related B-cell disorders are seen after organ transplantation and in congenital and acquired immunodeficiency states. Post-transplant lymphoproliferative disorders (PTLD) comprise a histologic spectrum ranging from hyper-plastic appearing lesions to frank non-Hodgkin's lymphoma or multiple myeloma histology. Multiple clones may co-exist, representing a uniquely different mechanism for lymphomagenesis. The incidence varies from 1% in renal recipients to 8% in lung recipients, but can be markedly increased by the use of anti-T-cell therapies, or by T-cell depletion in bone marrow transplantation. Pre-transplant EBV seronegativity increases risk to as high as 30%-50%. More than 90% of tumors are EBV-associated. Mechanisms for viral lymphomagenesis remain incompletely defined; LMP-1 may function as an oncogene and coprecipitates with TRAF, BCL-2 overexpression has also been identified. A possible direct tumorigenic effect has recently been suggested for cyclosporine. PTLD has a highly variable clinical picture, certain patterns are however seen. Reversibility of PTLD with reduction in immunosuppressives has long been recognized. Predicting reversibility has been difficult. The presence or absence of BCL-6 mutations has recently been identified as being of predictive value. Surgical resection can be curative. Cytotoxics, although problematic, can also be curative. Long term remission has been achieved with anti CD21 and CD24 antibodies; efficacy has been reported anecdotally for interferon alpha and for rituximab. In vitro expanded EBV-specific T cells have been effective as treatment and as prophylaxis in the setting of bone marrow transplantation. EBV viral load measured in blood appears to correlate with the emergence of PTLD and may facilitate prophylactic studies. PTLD is a model of immunodeficiency related EBV lymphomagenesis. Pathogenetic, therapeutic, and prophylactic insights gained from the study of PTLD are likely to be applicable to other immunodeficiency states and to EBV-related lymphoid neoplasia in general.