We investigated the serum concentration of interleukin-6 (IL-6) and the IL-6 family of cytokines (leukemia inhibitory factor (LIF), oncostatin M (OSM) and ciliary neurotrophic factor (CNTF) using an enzyme-linked immunosorbent assay (ELISA) in 64 patients with systemic lupus erythematosus (SLE) and 15 healthy controls. We also examined a possible association between the serum levels of these proteins and SLE activity as well as correlations between the IL-6 concentration and the levels of LIF, OSM and CNTF. IL-6 was detectable in all 64 patients with SLE and normal individuals, and the level of this cytokine was significantly higher in patients than in the control group (p < 0.002 ). LIF, OSM and CNTF were detectable in 9 (14.1%), 6 (9.4%) and 51 (78%) patients, respectively, and undetectable in the majority of healthy individuals. We found positive correlation between the serum concentrations of IL-6, LIF, OSM and CNTF and SLE activity. IL-6 and OSM serum levels were also correlated but not IL-6 and LIF or CNTF. In conclusion, an increase in the serum levels of IL-6 and, to a lesser extent of LIF, OSM and CNTF concentrations may be useful markers for SLE activity.