Th17-related cytokines (IL-17, IL-23, and IL-26) and receptors (IL-17R and IL-23R) were evaluated in MS patients under immunomodulatory IFN-β1 therapy during a 2year follow-up. Before the initiation of treatment, no significant difference was found in cytokine or receptor expression between controls and MS patients. Of the three cytokines evaluated, IL-26 was the highest in the patients' sera. The amount of IL-17 and CD13(+)IL-17R(+) cells was steadily decreased whereas IL-23 and IL-26 levels were gradually increased with IFN-β1 therapy. The patients in progressive phase had very high levels of IL-17. Th17-associated parameters should be considered in the immunomodulatory IFN-β1 therapy of MS.