In recent years, encephalopathy has increasingly been recognized as a complication of Hashimoto's thyroiditis. It can begin abruptly as a stroke-like event, acute seizures, or confusion, or as an insidious decline in cognitive function. Most reported cases have been on adult patients, although this encephalopathy does affect children as well. This form of encephalopathy should be considered in the differential diagnosis of children and adults with unexplained neurologic deterioration. We describe the case of a child in whom acute encephalopathy was the presenting symptom of Hashimoto's thyroiditis.