Three cases are described of a reversible encephalopathy, all presenting with marked neurological disturbance. In all three, the diagnosis was not clear at the time of presentation but eventually it was felt all of the cases were consistent with Hashimoto's encephalopathy. The diagnosis of Hashimoto's encephalopathy should be considered in any case of unexplained encephalopathy. Common features are high anti-thyroid peroxidase antibody titres, an abnormal EEG and an elevated CSF protein concentration. The encephalopathy is independent of thyroid hormonal status. Treatment with corticosteroids leads to a prompt resolution of symptoms and long-term low dose steroid therapy prevents further neurological recurrence.