We analyzed the titer of antithyroid autoantibodies (Abs) and thyroid function in 17 multiple sclerosis (MS) patients undergoing interferon-beta (IFN-beta) treatment and in 40 MS control patients. Basal evaluation revealed normal thyroid function in all patients. Abs were detected in 5 IFN-beta-treated patients (29%) and in 4 MS control patients (10%). Our results indicate that IFN-beta treatment may lead to thyroid autoimmunity. We therefore recommend periodic evaluations of antithyroid Abs and thyroid functionality in IFN-beta-treated MS patients.