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Background Recent evidence indicates that the slowly expanding population of CD5+ B cells that characterizes B-cell chronic lymphocytic leukemia (B-CLL) could be related to defects in the response to cytokine produced by T cells that regulate apoptosis. We studied the intracellular expressions of interleukin-2 (IL-2), tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) in T-helper 1 cells (Th1 response) of B-CLL. Methods Peripheral blood mononuclear cells from 21 healthy individuals and purified T cells from 21 early-stage and 15 late-stage B-CLL patients were activated with phorbol myristate acetate and ionomycin. The Th1 cytoplasmic cytokines were evaluated in CD4+ and CD8+ T cells by flow cytometry. Results The percentages of CD4+ and CD8+ T cells positive for IL-2 were significantly lower in B-CLL patients than in healthy individuals (P = 0.030 and 0.049, respectively). No significant differences in TNF-α or IFN-γ intracellular expressions were found between patients and healthy individuals. TNF-α- and IFN-γ–expressing CD8 T cells were disease stage dependent, being significantly higher in late-stage patients (P < 0.001 for both cytokines). Conclusions Our present observations suggested that Th1 cytokines may be of major importance in the pathogenesis of B-CLL. Cytometry Part B (Clin. Cytometry) 56B:23–29, 2003. © 2003 Wiley-Liss, Inc.