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Lymphoid neoplasia is a complex area comprising multiple diseases with varied pathology, treatment, and outcome. The non-Hodgkin's lymphomas are reviewed here. Non-Hodgkin's lymphomas, collectively, represent the sixth most common cancer in the United States as well as the sixth most common cause of cancer deaths. The overall incidence of non-Hodgkin's lymphoma has risen steadily over the past four decades. Although some of this is attributable to human immunodeficiency virus (HIV)-associated lymphoma, HIV-associated disease accounts for only a small part of the increase in lymphoma. As our knowledge of normal as well as neoplastic lymphoid development has expanded on the basis of histopathology as well as adjunct cellular and molecular techniques, multiple classifications have been proposed to take these into account. The clinical relevance to our understanding of non-Hodgkin's lymphoma is the concept that various lymphoid cancers are counterparts of stages of normal lymphoid development. Stages of lymphoid development in terms of cell surface markers and immunoglobulin gene rearrangements have been well characterized. These are particularly applicable to the early B-cell development, which is antigen-independent and occurs in the bone marrow. Diseases correlating with these stages are largely acute lymphocytic and lymphoblastic leukemia/lymphoma and high-grade lymphomas, such as Burkitt's lymphomas. Much has been learned recently about subsequent antigen-dependent B-cell development in secondary lymphoid organs to improve our understanding of the corresponding stages of B-cell neoplasia. Many of these stages correlate with more recently described entities such as mantle cell and marginal zone lymphomas. Histologic study remains crucial in determining the subtype of NHLs, whereas immunohistochemistry, surface phenotype, and molecular studies are useful in selected cases. Although some lymphoma classifications may be better in terms of understanding the lymphoma biology, the working formulation remains useful to guide clinical decision making. Lymphomas classified as low grade are considered incurable with standard therapy when diagnosed, as is usual, at advanced stages. Different subtypes may have different median survivals, but the goal has typically been palliation, whereas experimental approaches are clearly needed. Intermediate and high-grade lymphomas are potentially curable with aggressive combination chemotherapy. Recent evidence suggests that CHOP chemotherapy is as effective as more complex regimens. Still, 40% to 50% of patients are cured. Prognostic factor analysis has allowed separation of subgroups with much better survival in whom CHOP is adequate versus those with much poorer survival in whom experimental approaches are rational. Additional subtypes of lymphomas have been described and characterized since the working formulation was developed, including mucosa-associated lymphoid tissue tumors (MALT-oma), mantle zone lymphoma, anaplastic large cell lymphoma and AILD-like T-cell lymphoma. Approaches to these entities are still being optimized. Newer approaches, including high-dose therapy with stem cell support, biologic agents, and newer chemotherapeutic agents are discussed, as are special situations such as localized lymphoma of certain sites and lymphoma in immunosuppressed patients.