Hashimoto's encephalitis was first described more than 30 years ago. The clinical picture is typically that of a subacute encephalopathy with a moderate to marked alteration of consciousness, seizures, myoclonus or tremulousness. Additional stroke-like episodes can occur along the course of the disease which may be monophasic or relapsing. The diagnosis of Hashimoto's encephalitis requires the presence of an elevated titer of antithyroid antibodies (mainly anti-thyroperoxidase and also anti-thyroglobulin) which is not necessarily associated with obvious thyroid dysfunction. The results of neurologic investigations are not specific and show typically a global slowing of the EEG, a moderately high CSF protein content and a normal or near normal imaging except in rare cases. The disorder is considered autoimmune and is remarkably responsive to corticosteroids which must be started as soon as possible after the diagnosis has been confirmed biologically. The long-term prognosis is usually good but some patients may die or present major neurologic sequelae.