Leukaemia inhibitory factor (LIF) is a cytokine which possesses a wide range of biological activities including, like IL-6, the capacity to stimulate acute phase protein (APP) synthesis. We have developed a sensitive and specific ELISA for human LIF, and tested the circulating cytokine levels in various disease states, some of which are associated with inflammation. LIF was detected in 11/20 sera from patients with giant cell arteritis (GCA), a vasculitis syndrome affecting particularly the temporal artery, characterized by panarteritis with inflammatory cell infiltration. LIF levels were considerably elevated in some patients who also displayed elevated levels of IL-6 and C-reactive protein (CRP); however, no correlation was observed between the levels of circulating LIF and levels of IL-6 or CRP. Furthermore, LIF levels were not affected by corticosteroid therapy, whereas IL-6 and CRP decreased rapidly, as clinical symptoms resolved. A putative role for LIF in the persistence of histological lesions is discussed. This is the first report of the presence of circulating LIF in sera. These results are in agreement with the complexity of induced inflammatory cytokines and corticoid regulation of APP synthesis observed in vitro and in vivo.