Patients with Hashimoto's encephalopathy (HE), a steroid-responsive disorder, associated with Hashimoto's disease and high levels of thyroid-related autoantibodies usually present with a subacute onset of confusion, focal or generalized seizures. Frequent EEG abnormalities include generalized, rhythmic bifrontal or temporal slowing. Elevated protein levels or an intrathecal IgG synthesis may be present in cerebrospinal fluid (CSF). A 39-year-old woman underwent a relapsing course of myocloni and generalized seizures. Initially, thyroid function, thyroid-related autoantibody screening and cerebral MRI were unrevealing. CSF showed oligoclonal bands. Short-term treatment with high doses of prednisolone resolved the myocloni. During the 5th episode of myocloni, signs of hyperthyroidism and elevation of thyroid microsomal antibody titer developed. Hashimoto's thyroiditis and HE were diagnosed. After subtotal thyroidectomy the patient remained asymptomatic.