Summary Chronic lymphocytic leukaemia (CLL) cells encounter T‐cells and proliferate in response to T‐cell signals in the lymph node microenvironment. In this report we determined interleukin 21 (IL21) function in CLL and showed that IL21 and interleukin 4 (IL4) act co‐operatively to promote leukaemic cell proliferation without apoptosis or differentiation We further show that IL21 increased side population (SP) cells, which are associated with resistance to chemotherapy and increased self‐renewal capacity in CLL. IL21 and IL4 are the major cytokines produced by the recently described CD4+ T follicular helper (Tfh) cell subset. Determination of Tfh cells in peripheral blood showed that patients had significantly increased numbers as compared to normal subjects although no association was found between Tfh numbers and IGHV gene mutational status or clinical stage. Our data suggests that the Tfh cytokines, IL4 and IL21, contribute to driving leukaemic cell proliferation in the lymph node microenvironment, and may contribute to the specific production of cells resistant to conventional chemotherapy. We suggest that increased circulating Tfh cells is a component of T‐cell dysregulation in CLL. Our findings have implications for the therapeutic use of IL21.