A 4-year-old female patient with epilepsy with continuous spike-and-waves during slow-wave sleep not classified as Landau-Klefner syndrome, refractory to antiepileptic drugs including valproate, benzodiazepines, and lamotrigine, was treated successfully with high-dose intravenous methylprednisolone therapy. Valproate, clobazam, and lamotrigine were continued at the same dose during and after high-dose intravenous corticosteroid therapy. During corticosteroid therapy, awake and sleep electroencephalogram was recorded every day. On day 7, a dramatic clinical and electroencephalographic response was observed. After high-dose intravenous methylprednisolone, prednisolone was administered orally (2 mg/kg daily) for 2 months, then gradually withdrawn. After the withdrawal of corticosteroid therapy, the patient maintained the clinical improvement in behavior, and no continuous spike-and-wave electrical status epilepticus during slow-wave sleep occurred on routine monthly sleep electroencephalogram performed for the last 6 months. In the present case, an add-on high-dose intravenous corticosteroid seems to be effective in the treatment of patients with electrical status epilepticus during slow-wave sleep syndrome, especially when antiepileptic drugs fail.