Hashimoto's thyroiditis (HT) is the most common disorder affecting the thyroid gland. Encephalopathy associated with abnormal thyroid functions, such as myxedema encephalopathy, is treatable. Hashimoto's encephalopathy (HE) was recognized as a new clinical disease based on an autoimmune mechanism associated with HT, and steroid treatment has been successfully administrated. Recently, we discovered serum autoantibodies against the NH2-terminal of a-enolase (NAE) as a specific diagnostic marker for HE. We analyzed these serum anti-NAE autoantibodies and the clinical features in 84 cases of HE. The 84 patients consisted of 26 men and 58 women, from many institutions throughout Japan and other countries. A total of 37 patients carried anti-NAE antibodies (44%). The age was widely distributed between 19 and 87 years old, with two peaks (around 20-30 and 50-70 years old). Most patients were in euthyroid states, and all patients had anti-thyroid (TG) and/or anti-thyroid peroxidase (TPO) antibodies, and anti-TSH receptor (TSHR) antibodies in some cases. Only 20% of patients had past histories of HT. The acute encephalopathy form was the most common clinical feature, and subacute or chronic psychiatric forms and other forms such as pure ataxia, limbic encephalopathy, and Creutzfeldt Jakob-like forms followed. The patients with anti NAE antibodies tended to exhibit acute encephalopathy. The most common neuropsychiatric features were consciousness disturbance, psychiatric symptoms, and seizures. Involuntary movements (tremor, myoclonus, or choreoathetosis) or ataxia occasionally occurred. Abnormalities, especially the slowing of background activities, on EEG and elevated levels of protein/IgG in cerebrospinal fluid (CSF) were common and useful laboratory findings for the diagnosis, while abnormalities on brain MRI were rare and non-specific in HE. Immunotherapies such as glucocorticoid, immunosuppressants, immunoglobulin, and plasma exchange, were recommended and effective for HE treatment. HE belongs to a part of a clinical spectrum consisting of individuals with anti-thyroid antibodies, overlapping the clinical spectrum of HT. Anti-NAE autoantibodies were positive in 44% of patients with HE. Considering the overall findings, we should be aware of the possibility of autoimmune encephalopathy associated with thyroid disorders (HE) in patients with an unknown etiology of neuronpsychiatric symptoms with/without a past history of HT.