Objectives-During an epidemiological study of Creutzfeldt-Jakob disease in Germany, Hashimoto's encephalitis was encountered as a diVerential diagnosis, which has not yet been described in this context. Methods-The symptoms and findings of seven patients who fulfilled the criteria for "possible"Creutzfeldt-Jakob disease are presented. Results-A Hashimoto's thyroiditis with antibodies against thyroglobulin or thyroid peroxidase, or both and a hypoechoic thyroid ultrasonogram were found in all cases. Analysis of CSF disclosed an increased leucocyte count in three patients, and a raised CSF:serum concentration ratio of albumin (QAlb) in four patients. The 14-3-3 protein, typical of Creutzfeldt-Jakob disease, could not be detected in any of our patients. No periodic sharp wave complexes, which are typical of Creutzfeldt-Jakob disease, were detected on EEG in any of the cases. By contrast with Creutzfeldt-Jakob disease, which leads to death within a few months, the patients with Hashimoto's encephalitis often recover quickly when treated adequately. All the patients improved after administration of corticosteroids. Conclusion-The clinical symptomatology of both diseases may be very similar: dementia, myoclonus, ataxia, and personality change or psychotic phenomena are characteristic symptoms. (J Neurol Neurosurg Psychiatry 1999;66:172-176) Keywords: Hashimoto's encephalitis; Hashimoto's thyroiditis; Creutzfeldt-Jakob disease; encephalopathyIn the course of our epidemiological study of Creutzfeldt-Jakob disease in Germany, we encountered Hashimoto's encephalitis as a differential diagnosis, which has not yet been described in this context. Hashimoto's encephalitis is associated with Hashimoto's thyroiditis.2 As yet, no data are available on the incidence of this disease; it is much more often found in women than in men. The most common symptoms are myoclonus, epileptic seizures, dementia, and disturbances of consciousness. In most cases therapy with corticosteroids leads to clear improvement or even disappearance of the symptoms.Chronic lymphocytic Hashimoto's thyroiditis is an often occurring (3%-4% of the population) organ specific autoimmune disease accompanied by development of goitre.29 Middle aged women are most often aVected. About one third of the patients are hypothyroid. In almost all cases antibodies against thyroglobulin, or thyroid peroxidase, or both are detectable. [30][31][32] Another typical feature is a hypoechoic thyroid ultrasonogram. Fine needle biopsy discloses inflammatory infiltrates with lymphocytes, plasma cells, colloid accumulation, and cell detritus.Creutzfeldt-Jakob disease is characterised by rapidly progressive dementia, myoclonus, pyramidal and extrapyramidal signs, and cerebellar symptoms.3 It is a rare disease (with an incidence of about one case/1000 000 population/ year), usually occurring in persons aged 60-70 years. Creutzfeldt-Jakob disease is also more often seen in women than in men. In most cases death occurs within a few months after onset. The clinical suspicion ...