Rheumatoid arthritis is characterised by the interaction of multiple mediators, among the most important of which are cytokines. In recent years, extensive data demonstrates a pivotal role for one cytokine, macrophage migration inhibitory factor (MIF), in fundamental events in innate and adaptive immunity. MIF has now been demonstrated to be involved in the pathogenesis of many diseases, but in the case of RA the evidence for a role of MIF is very strong. MIF is abundantly expressed in the serum of RA patients, and in RA synovial tissue where it correlates with disease activity. MIF induces synoviocyte expression of key proinflammatory genes including TNF, IL-1, IL-6, IL-8, cPLA2, COX2 and MMPs. MIF also regulates the function of endothelial cells and B cells. Moreover, MIF is implicated in the control of synoviocyte proliferation and apoptosis via direct effects on the expression of the tumor suppressor protein p53. In multiple rat and mouse models of RA, anti-MIF antibodies or genetic MIF deficiency are associated with significant inhibition of disease. MIF ^-/- mice further demonstrate increases in synovial apoptosis. That the human Mif gene is encoded by different functional alleles in subjects with inflammatory disease also provides evidence for the role of MIF in RA. The mechanism of action of MIF is becoming better understood. MIF appears to interact with cell surface CD74, with consequent activation of MAP kinases but possibly not NFκB intracellular signal transduction. This apparent selectivity may be implicated in the ability of MIF to antagonise the effects of glucocorticoids. As MIF expression is induced by glucocorticoids, inhibition of its antagonistic effects may permit enhanced therapeutic effect of glucocorticoids, or "steroid sparing". To date there are no clinical trials of MIF antagonism in any disease, but exploitation of antibody, soluble receptor, or small molecule approaches enabled by the unique crystal structure of MIF, may soon lead to the ability to test in the clinic the importance of this cytokine in human RA.