Abstract: Analysis of cytokine production is a tool to functionally characterise T cells. In this study, spontaneous and polyclonal activation induced cytokine production in T cells were assessed by flow cytometry in patients with B‐CLL. Patients with progressive disease had a significantly increased number of T cells spontaneously producing IL‐2, IL‐4 and GM‐CSF as compared to healthy donors and patients with non‐progressive CLL, which was not the case for TNF‐α and IFN‐γ producing T cells. However, no difference in the frequency of T cells producing these cytokines was seen comparing patients with non‐progressive disease to control donors. Polyclonal activation of B‐CLL T cells in vitro induced an increased proportion of T cells producing these five cytokines in patients as well as in control donors, indicating that T cells in CLL patients might have a relatively well preserved functional capacity. However, the increase in GM‐CSF, TNF‐α and IL‐4 producing T cells was more marked in CLL patients than in controls. Furthermore, following activation, a higher frequency of cytokine‐producing T cells was noted in patients with progressive disease as compared to those with non‐progressive disease. The augmented number of cytokine‐producing T cells in CLL may indicate an up‐regulated capability of T cells to secrete cytokines, especially in patients with progressive CLL. The increased production of the T cell derived cytokines GM‐CSF, TNF‐α, IL‐4 and IL‐2 is interesting, as these cytokines have previously been shown to support growth of B‐CLL leukaemic cells in vitro and as T cells might specifically recognise the autologous leukaemic B cells in vivo. The findings may suggest a role for T cells in the pathogenesis of B‐CLL.