A 51-year-old man with a past history of hypothyroidism suddenly became comatose after a few days of general malaise and headache. On admission to our hospital, his consciousness level was Japan Coma Scale III-200, but no focal neurological deficits were evident. Serum anti-thyroglobulin antibody was >4,000 IU/ml and anti-thyroid peroxidase antibody was 265 IU/ml. Well characterized neuronal antibodies were not fully examined in this case, but based on high titers of serum thyroid antibodies, methylprednisolone pulse therapy was started under diagnosis of suspected Hashimoto encephalopathy. Although methylprednisolone pulse therapy was effective, every time the dose of oral prednisolone was reduced, relapse attacks similar to the first episode occurred, a total of seven times. At each relapse, cerebrospinal fluid findings and MRI findings were normal. Plasmapheresis and azathioprine were added, until two years after onset, when further acute neurological attacks no longer occurred, but attention and memory impairments persisted. Relapse in Hashimoto encephalopathy is not rare; careful, long-term follow-up is needed.