Chronic lymphocytic leukemia is a hematologic neoplasm characterized by proliferation and accumulation of mature-appearing lymphocytes. Most cases involve a clonal proliferation of B lymphocytes. The cells typically have low levels of surface immunoglobulin; usually mu or mu and delta heavy chains, and either kappa or lambda light chains. The cells also show receptors for mouse erythrocytes, Fc receptors for lgG, complement receptors, la antigens, and B-cell-associated antigens. Although chronic lymphocytic leukemia is usually a stable disease over months to years, transformation of both clinical and biological features may occur. Prognostic factors include the leukemia cell count (greater than 40 X 109/L), anemia, thrombocytopenia, chromosome abnormalities, and the pattern of bone marrow involvement. Alkylating agents, radiation therapy, and corticosteroids are commonly used to treat patients with chronic lymphocytic leukemia. Although these agents are useful, few data show that survival has been substantially improved. Recently, biological response modifiers such as monoclonal antibodies and interferon have been studied.