Lymphoid proliferations in the acquired immunodeficiency syndrome reflect the profound immunologic imbalances induced by the lymphotropic human immunodeficiency virus, and comprise a range of lymphoproliferative disorders from benign lymphadenopathy to malignant lymphoma. Persistent generalized lymphadenopathy (PGL) is associated with a frequently debilitating complex of clinical symptoms and waxing and waning lymphadenopathy. Biopsies of lymph nodes, bone marrow, and other involved tissues show a spectrum of abnormal lymphoid proliferations, with eventual lymphoid depletion, lymphadenopathic Kaposi's sarcoma, and malignant lymphoma. Although individual features of AIDS-related lymphadenopathy may not be specific, the constellation of histologic, immunologic, and ultrastructural findings is highly characteristic of the disorder and useful as a predictor of clinical course. Malignant lymphomas that develop within this setting of multicloncal B cell expansion and impaired immune surveillance have distinctive clinical, histologic, and molecular biologic parameters.