B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of mature-appearing clonal B cells exhibiting coexpression of CD5 and CD23. In addition to the accumulation of neoplastic B cells, numerous T-cell abnormalities also occur in B-CLL patients. In this study, the presence, and distribution within the T-cell subsets, of clonal/oligoclonal T cells was studied. Multicolor flow cytometric techniques were employed using combinations of anti-CD3, anti-CD4, and anti-CD8 antibodies coupled with antibodies specific for V โฃ and V โค T-cell receptor (TCR) epitopes. Molecular studies of TCR gene sequences were done to confirm the presence of clonal/oligoclonal T-cell populations. In the flow cytometric studies, examination of V โฃ /V โค expression found evidence of clonal/oligoclonal expansion in 9 of 19 patients studied. In eight of the nine patients, the expansions were restricted to the CD3 ุ CD8 ุ cell population. Molecular analyses were performed in 16 patients, 12 of whom showed a clonal or oligoclonal pattern. Of the four patients who were negative in the molecular analyses, all demonstrated flow cytometric evidence of clonal/oligoclonal expansions. Thus, when the flow cytometric and molecular analyses were considered together, all 16 patients for whom parallel analyses were done showed evidence of clonal/oligoclonal expansions. These results confirm previous work demonstrating that the majority of B-CLL patients harbor clonal/oligoclonal expansions within the T-cell population. Additionally, based on the relative numbers of cells expressing specific V โฃ or V โค epitopes, these results show that these expansions occur primarily within the CD3 ุ CD8 ุ T-cell population. Cytometry (Comm. Clin. Cytometry) 42:188 -195, 2000.