High-grade B-cell non-Hodgkin's lymphomas are observed in 5% to 10% of patients with acquired immunodeficiency syndrome. To describe their histologic subtypes, a group of pathologists was formed. One hundred thirteen cases were reviewed and classified according to the Working Formulation, the updated Kiel classification, and a recent description of morphologic variants of high-grade B-cell non-Hodgkin's lymphoma. Three major types of intermediate- or high-grade lymphomas were observed: (1) large-cell or centroblastic mainly polymorphic lymphomas with a component of immunoblasts (35 cases); (2) immunoblastic lymphomas with plasmablastic and plasmacytic features in most cases (33 cases); and (3) small non-cleaved cell Burkitt's or non-Burkitt's lymphoma (41 cases), with 15 cases fitting typical criteria of Burkitt's lymphoma and 26 heterogeneous cases in which the size and shape of the cells and the presence of plasmablastic features varied. The most frequent pathologic sites of involvement at presentation were the lymph nodes, gastrointestinal tract, bone marrow, brain, oral cavity, and muscles. A comparison between the histologic type and the site of involvement showed that most cases involving lymph nodes, bone marrow, or muscles were small noncleaved cell Burkitt's or non-Burkitt's lymphomas, while those that affected the gastrointestinal tract, brain, and oral cavity were centroblastic or immunoblastic lymphomas with consistent plasmacytic differentiation. In 10 cases, previous persistent generalized lymphadenopathy syndrome was present. In 13 cases, the lymphomatous proliferation was associated with follicular or diffuse hyperplasia seen on the same lymph node biopsy specimen or in another lymph node.