Characteristic clinical findings of Hashimoto's encephalopathy (HE) are stroke-like episodes, epileptic seizures, myoclonus, psychosis, and progressive cognitive impairment. Diagnosis of HE is supported by elevated antithyroid antibodies, an abnormal EEG, and by good response to steroids. We report on a 74-year-old female patient with a severe depressive episode who showed no treatment response to citalopram 40 mg/day and venlafaxine 150 mg/day. Diagnostic examination revealed an abnormal EEG, elevated thyroid peroxidase antibodies (TPO-Ab), and older postinflammatory changes in thyroidal sonography. We diagnosed a depression in HE and began treatment with prednisolone 70 mg/day with stepwise dose reduction, continuing treatment with venlafaxine 150 mg/day. Within 4 weeks of treatment, the severe depressive episode disappeared as well as abnormal EEG. In addition, serum values of TPO-Ab decreased. In HE, depressive symptoms can possibly be seen in a subgroup of patients or in the early course of the disease. Diagnosis of HE should be included in diagnostic procedures in cases of therapy-refractory depression because of a good response of HE to steroids.