HIV-related non-hodgkin lymphomas currently occur in 5 to 8% of AIDS patients. AIDS-related lymphomas are high-grade tumors with the morphologic characteristics of either small noncleaved cell lymphomas of the Burkitt type or large cell centroblastic and immunoblastic lymphomas. Mixed features may be found, making classification difficult. Useful methods for characterizing AIDS-related non-hodgkin's lymphomas include immunophenotypic studies using B-cell differentiation and activation antigens (HLA-DR, CD10, CD19, CD20, CD21, CD22, CD23, CD38), evaluation of expression of surface immunoglobulins (IgS), activation and proliferation (CD25, CD30, CD71, Ki67), and identification of T-cell markers (CD1, CD2, CD3, CD4, CD5, CD7, CD8). Cases studied were of the B-cell type. Comparison with morphologic features revealed that Burkitt's lymphomas were monoclonal and expressed B-cell markers (CD10, CD19, CD20, CD22, CD38) and surface immunoglobulins, especially IgM kappa. This immunophenotype is similar to that of large cell or centroblastic non-hodgkin's lymphomas, suggesting that Burkitt lymphomas originate from centrofollicular cells. Immunoblastic non-hodgkin's lymphomas were monotypic or polytypic and expressed CD10 and CD38 antigens but not the other B-cell antigens Furthermore, a very large number of cells stained positively with the Ki67 antibody demonstrating that most lymphoma cells were undergoing cycling.