Hashimoto's encephalopathy (HE) is a rare autoimmune disease characterized by a high serum concentration of antithyroid antibodies without evidence of cerebral disease. Magnetic resonance imaging (MRI) findings in HE patients are nonspecific, although diffuse or focal white matter changes have been reported in several cases. We present a rare case involving a 79-year-old woman with elevated antithyroid antibody levels and abnormal imaging findings similar to meningoencephalitis. Serial MRI initially showed multiple T2 hyperintense lesions with diffuse leptomeningeal enhancement that disappeared after steroid therapy.