Hashimoto's encephalopathy is a poor understood neurological disorder associated with thyroid disease. We describe a 12 years old woman with a relapsing clinical picture characterized by acute onset confusion, tremor, convulsive seizures and a stroke-like episode associated with autoimmune thyroid disease. High titers of antithyroid antibodies were detected in her serum. Neurologic investigation showed a marked slowing of the basic activity in EEG, diminished perfusion of left hemisphere in brain SPECT, mild CSF protein level without pleocytosis, and normal brain CT and cerebral angiogram. The clinical course was favorable without corticosteroid therapy. Antithyroid antibodies and EEG were normal after one-year follow-up. We suggest that antithyroid antibody levels should be checked in any unexplained acute or subacute encephalopathy, especially when a relapsing course, stroke-like exacerbation, seizures, tremor or elevation of the CSF protein are found.