Clinically isolated syndrome (CIS) suggestive of multiple sclerosis (MS) is the earliest clinically evident phase of the disease, which may provide valuable insight into the molecular mechanisms of the initiation of the autoimmune response in MS. Our results introduce IL-11 as a new cytokine that plays a role in the autoimmune response in the early phase of the disease. IL-11 is the highest upregulated cytokine in the sera and cerebrospinal fluid from CIS patients, which is also increased in patients with clinically definitive relapsing-remitting MS in comparison with healthy control subjects. Serum IL-11 levels are significantly increased during clinical exacerbations in comparison with remissions in the same patients. CD4(+) cells represent a predominant cell source of IL-11 in the peripheral circulation, and the percentage of IL-11(+)CD4(+) cells is significantly increased in CIS patients in comparison with healthy control subjects. Furthermore, we have identified IL-11 as a new Th17-promoting cytokine, because it induces a differentiation of naive CD4(+) T cells into Th17 cells, as well as expansion of Th17 memory cells. Because the Th17 cytokines IL-17F, IL-21 and TNF-α, and TGF-β induce differentiation of naive cells in the IL-11-secreting CD4(+) cells, we propose that cross-talk between IL-11(+)CD4(+) and Th17 cells may play a role in the inflammatory response in relapsing-remitting MS.