In patients with chronic lymphocytic leukemia (CLL), effector and memory CD4 + and CD8 + T cells are expanded. We investigated whether these CLL specific T-cell expansions also occur in monoclonal B lymphocytosis (MBL), the pre-leukemic state of CLL, which is currently distinguished from CLL by an arbitrarily chosen cut-off value of CD19 of 5.0 × 10(9)/L. Whereas an increase in effector and memory CD4 + and CD8 + T cells was found in CLL, these expansions could not be found in MBL. Although a significant correlation was found between absolute B cell count (CD19) and T cell numbers, correlation coefficients were rather low. Therefore, we analyzed whether an optimal threshold for CD19 number could be defined which best related to an expansion of T cells. The B-cell threshold that best predicted expansion of CD3 +, CD4 + and CD8 + T cells, respectively, was 10 × 10(9)/L. Our study indicates that a higher cut-off value than the current 5.0 × 10(9)/L relates better to the biological impact of CLL.