Peripheral blood (PB) T cells from 56 patients with B-cell chronic lymphocytic leukemia (B-CLL) were analyzed by two- and three-color immunofluorescence (IF) to determine the expansion of distinct T-cell subsets and their relationship with the clinical and biological features of the disease. We detected the expansion of an unusual T-cell subpopulation expressing lower CD4 or CD8 levels (CD4lo, CD8lo) than classic T cells (CD4hi, CD8hi). This subpopulation also expressed low levels of the CD3/TCR alpha/beta complex and was CD19-CD13-CD14-. A phenotypic analysis probing the activation level of CD4lo, CD8lo, CD4hi, and CD8hi cells showed that they comprised increased counts of HLA-DR+, CD11b+, CD45R0+, and CD45RA+ cells. Subset expansion ranged from 2.1- to 13.6-fold. Statistical analysis showed that the size of some of these subsets was correlated to intrinsic features of the tumor. First, CD4loHLA-DR+ cell counts were higher in patients with stage A than those with stages B and C disease. Second, CD8loHLA-DR+ cell counts were higher in patients in stable remission than in those at diagnosis. Third, CD4loHLA-DR+, CD4loCD45R0+, CD4loCD45RA+, and CD4hiCD11b+ cell counts were higher in patients whose tumor cells expressed high levels of surface immunoglobulin (sIg) than in those expressing low levels. The involvement of CD4lo and CD8lo cells in most of these correlations suggests that they may be tumor-reactive cells. Similar cells described in human and murine autoimmune disease have been shown to be autoreactive anergic cells, which may derive from nonclassic pathways of T-cell development.