Hashimoto encephalopathy (HE) has been recognized as a new clinical disease based on an autoimmune mechanism associated with Hashimoto's thyroiditis. HE is successfully treated with steroids. In 2005, we discovered serum autoantibodies against the NH2-terminal of α-enolase (NAE) as a highly specific diagnostic biomarker for HE. We analyzed the serum anti-NAE autoantibodies and clinical features in many cases of HE from institutions in Japan and other countries. Approximately half of the patients with HE had anti-NAE antibodies. In our study, HE was widely distributed in patients aged 50-60 years. Most patients with HE were in euthyroid states, and all patients had anti-thyroid antibodies. The common neuropsychiatric features of these patients were consciousness disturbance and psychosis, followed by cognitive dysfunction, involuntary movements, seizures, and ataxia. Abnormalities in EEG and decreased cerebral blood flow on the brain SPECT were common findings. In contrast, abnormalities on the brain MRI were rare except for diffuse subcortical lesions and limbic lesions. Patients with HE had varied clinical phenotypes, including acute encephalopathy, chronic psychiatric form, and other particular clinical presentations, such as limbic encephalitis, progressive cerebellar ataxia, and Creutzfeldt-Jakob disease (CJD)-mimic form. The cerebellar ataxic form of HE clinically mimics spinocerebellar degeneration (SCD) and is characterized by the absence of nystagmus, absent or mild cerebellar atrophy, and lazy background activities on electroencephalography (EEG). Taken together, these features should indicate the possibility of encephalopathy associated with thyroid disorders.