Although imatinib mesylate (IM) is highly effective at inducing complete cytogenetic remission in patients with chronic myelogenous leukemia (CML), it is known to suppress T-cell proliferation in vitro. As cytokines are required for T-cell proliferation, we investigated the effects of IM on cytokine synthesis by T cells of CML patients by assessing cytokine synthesis by activated CD4+ and CD8+ T cells in vitro. The activation of T cells in the whole blood of IM-treated patients (CML-IM) with Staphylococcus enterotoxin B resulted in significantly lower percentages of CD4+ T cells that synthesized interleukin 2 (P = 0.017), interferon-gamma (P = 0.010), and tumor necrosis factor-alpha (P = 0.009) than did the activated T cells of control subjects. The addition of exogenous IM to the cultures of peripheral blood mononuclear cells of CML-IM patients reduced Th1 cytokine synthesis by the CD4+ T cells. Furthermore, IM therapy at clinical doses suppressed the tyrosine phosphorylation of ZAP70. These findings suggest that inhibition of ZAP70 signaling pathway and suppression of Th1 cytokine synthesis by CD4+ T cells required the presence of IM at the time of T-cell activation through the T-cell receptor.