Hashimoto's encephalopathy may present with a wide variety of different neurological symptoms and signs. These include recurrent severe migrainous headache, psychoses, seizures, ataxia, dementia, stupor and coma. We present a personal series of 18 adult patients with Hashimoto's encephalopathy and a review of the literature in this paper. The natural history, laboratory abnormalities and neuroimaging data in these cases favour an immunopathological basis for this syndrome similar to relapsing acute disseminated encephalomyelitis. We suggest that Hashimoto's encephalomyelitis should be considered in the differential diagnosis of seizures, coma, atypical migraine and reversible dementia. Serological screening for anti-thyroid antibody should form part of the initial investigations in all relapsing and reversible encephalopathies.