Nonconvulsive status epilepticus usually presents with altered mentation without distinct manifestations of seizures. It may be related with various medical disorders. Hashimoto's encephalopathy is characterized by various neurological manifestations accompanied by high titers of anti-thyroid antibodies. Here, we report a patient with nonconvulsive status epilepticus caused by Hashimoto's encephalopathy who showed a dramatic response to steroids.