Hashimoto's encephalopathy is diagnosed when patients exhibit features of corticosteroid-responsive encephalopathy and positive antithyroid antibodies. The relationship between antithyroid antibodies and encephalopathy is subject to considerable debate. We describe corticosteroid-responsive encephalopathy in a 14-year-old boy with positive antimicrosomal antibodies. His history included subtle neurocognitive decline. He presented with seizures. He underwent a brain biopsy before initiating treatment after his third episode. That biopsy was consistent with central nervous system vasculitis. This report is unique because, to our knowledge, it describes the first pretreatment brain biopsy of a pediatric patient who fits the criteria for Hashimoto's encephalopathy.