In multiple myeloma (MM), impaired apoptosis is critical for clonal expansion of malignant MM cells in the bone marrow (BM), although the precise underlying mechanism has not been defined. During physiologic plasma cell differentiation, clonal expansion of antigen-activated B cells in the germinal center requires CD40 signalling, which mediates CD40L signals from activated T cells to promote B cell activation and survival. Terminal differentiation of plasma cells from activated B cells is then accompanied by a progressive reduction of CD40 expression, suggesting that the life span of normal plasma cells may be limited by the loss of CD40L signalling. Here we show that CD40L and CD40 are both aberrantly expressed in primary BM MM cells, but not in normal BM plasma cells, their likely normal counterparts. CD40L is also copiously expressed in multiple cell types in myeloma and normal bone marrows, indicating that autocrine and paracrine CD40L signalling may prolong the survival of MM cells in the bone marrow microenvironment. Supporting this hypothesis, disruption of CD40L signalling by an antagonizing CD40L antibody markedly accelerates apoptosis of BM MM cells ex vivo. In the majority of MM cases (N=50), CD40 is expressed at either greater than 90% of BM MM cells or lower than 30% of BM MM cells in vivo, but rarely in between. However, there is no apparent correlation between the frequency of CD40 expression in BM MM cells in vivo and immunoglobulin isotype or treatment history, including bortezomib therapy. Ex vivo, the level of CD40 expression in BM MM cells is enhanced in response to bortezomib, which rapidly activates CD40 transcription. Moreover, killing of primary BM MM cells by bortezomib ex vivo is attenuated by CD40L signalling, at least in part through sustaining NF-?B activation that is required for MM cell survival. Together, our findings suggest that autocrine and paracrine CD40L signalling is critical for the survival of MM cells in the bone marrow and may contribute to the development of drug resistance.