Interleukin-1 (IL-1) is one of the major proinflammatory cytokines expressed consistently in multiple sclerosis (MS) lesions. Interleukin-1 receptor antagonist (IL-1ra) is the only known naturally occurring specific antagonistic cytokine counteracting IL-1. Thus IL-1ra may have a downregulating potential in the disease course of MS. We analysed if circulating IL-1ra could be associated with different disease stages of MS in sera of 84 MS patients and 18 controls. IL-1ra showed considerable variations in MS patients and controls. Nevertheless we found significantly elevated serum levels in active as well as in stable disease stages compared to controls. IL-1ra levels were higher in progressive disease courses compared to relapsing-remitting MS, but not statistically significant (median: 516 versus 434 pg/ml). Further analysis with larger groups of patients and longitudinal studies will clarify if IL-1ra is useful as a prognostic serum marker in MS.