Not all patients with clinically isolated syndrome (CIS) suggestive of multiple sclerosis (MS) develop clinically defined MS (CDMS). At first clinical event we observed increased production of IL17, IFNgamma and IL10 by peripheral blood mononuclear cells from patients with CIS that remained high in remission. In CD4+ T cells pSTAT3 expression was higher in patients who subsequently converted to CDMS than in patients who did not and controls. The persistency of high levels of pSTAT3 in circulating CD4+ T cells from CIS patients after the first clinical event may favor the early conversion to CDMS.