This report discusses a case of T cell chronic lymphocytic leukemia (T- CLL) in an elderly white man whose lymphocytes expressed a post-thymic phenotype except for the coexpression of T4 and T8 on 80% to 95% of the cells. Because of the uncommon phenotype, in vitro functional assays were performed that showed decreased mitogenic responses but normal helper activity for B cell immunoglobulin secretion and normal suppressor activity of lectin-induced mitogenesis. Morphologic evaluation by both light and electron microscopy and cytochemical staining were consistent with the “knobby” type T-CLL. Adenosine deaminase and terminal deoxynucleotidyl transferase (TdT) levels were low, but the acetylcholinesterase level was normal, which is consistent with the peripheral T cell phenotype. The patient underwent splenectomy, and the spleen cells showed very low levels of T3 and T4 by immunoperoxidase and undetectable levels by immunofluorescence. The morphology of the splenic infiltrate was not significantly different from that in the initial bone marrow. Human T cell leukemia virus (HTLV) antigen and antibody tests were negative. The cells in this leukemia apparently are derived from a transitional stage of maturation between the cortical and medullary thymocyte. A small subset of lymphocytes of identical phenotype to this leukemia has been identified in normal individuals.