The incidence of lymphoproliferative disease is significantly higher in individuals who have congenital, acquired, or iatrogenically induced immunodeficiency. The immunodeficiency-associated lymphoproliferative disorders are clinically and pathologically heterogeneous, are of variable clonal composition, and vary according to the immunodeficiency syndrome. Nonetheless, they share several features, including frequent origination in or involvement of extranodal sites, diffuse aggressive histology, B-cell lineage derivation, association with the Epstein-Barr virus (EBV), and, often, rapid clinical progression. Reactive and atypical lymphoid hyperplasias and malignant lymphomas occur in association with congenital (primary) immunodeficiency. Post-transplantation lymphoproliferative disorders are often comprised of a polymorphic cell population, making it difficult to identify their benign or malignant nature by histopathologic criteria alone. Recent studies suggest that they are divisible into plasmacytic hyperplasias, polymorphic lymphoproliferative disorders, and malignant lymphomas. The plasmacytic hyperplasias are polyclonal and generally regress spontaneously following withdrawal of immunosuppression. The malignant lymphomas are monoclonal, possess a variety of genetic alterations, and generally progress despite aggressive therapy. The polymorphic lymphoproliferative disorders are also monoclonal but display variable clinical behavior, their progression apparently correlating with bcl-6 gene mutation. Non-Hodgkin's lymphoma (NHL) is the second most common AIDS-related neoplasm and an AIDS-defining illness. AIDS-related NHLs are divisible by anatomic site of origin into systemic (nodal/extra nodal), primary central nervous system, and body cavity-based (primary effusion) lymphomas; and by histopathology into Burkitt's and Burkitt's-like lymphoma, large cell lymphoma, and large cell immunoblastic (plasmacytoid) lymphoma More than 90% are monoclonal B-cell neoplasms. The primary effusion lymphomas contain the Kaposi's sarcoma-associated herpesvirus. Multiple molecular pathways appear to operate in AIDS lymphomagenesis and some may be preferentially associated with specific histopathologic categories or anatomic sites of origin. In conclusion, the immunodeficiency-associated lymphoproliferative disorders often represent a significant diagnostic problem requiring correlative analysis of the clinical behavior of the patient with the histopathology, immunophenotype, clonal composition, viral content, and genetic alterations of the lymphoproliferative disorder. They also represent an important biological model for studying the development and progression of lymphoid neoplasia