T helper type 1(Th1)/Th2 paradigm has been expanded by discovery of a novel effector T cell (T_eff) subset, Th17 cells, which produce a proinflammatory cytokine IL-17. Th17 cells have recently been shown to play a major role in numerous autoimmune diseases that had previously been thought to be Th1-dominant diseases. We here studied the significance of Th17 cells in iodine-induced autoimmune thyroiditis in nonobese diabetic-H2^h4 mice, a mouse model of Hashimoto's thyroiditis in humans, which spontaneously develop antithyroglobulin autoantibodies and intrathyroidal lymphocyte infiltration when supplied with iodine in the drinking water. We observed increased numbers of Th1 and Th17 cells in spleen and accumulation of both types of T_eff in the thyroid glands of iodine-fed wild-type mice, indicating that Th17 cells as well as Th1 cells constitute thyroid lesions. Furthermore, the incidence and severity of intrathyroidal lymphocyte infiltration, and the titers of antithyroglobulin autoantibodies were markedly reduced in iodine-treated IL-17^−/− mice as compared with wild-type mice. Of interest, IL-17^+/− mice showed an intermediate phenotype. Therefore, the present study, together with a previous report demonstrating the importance of Th1, not Th2, immune response for developing thyroiditis using mice deficient for interferon-γ or IL-4, clearly indicates that both Th1 and Th17 cells are critical T_eff subsets for the pathogenesis of spontaneous autoimmune thyroiditis in nonobese diabetic-H2^h4 mice.