To assess the efficacy and tolerability of steroids in epileptic syndromes with continuous spike-waves during slow-wave sleep (CSWS), charts of 44 children (25 boys) who received corticosteroids for cognitive and/or behavioral deterioration associated with CSWS were retrospectively reviewed. Awake and sleep electroencephalography (EEG) records, clinical and neuropsychological assessments were available before, during, and after corticosteroid therapy. Evaluation focused on effects on EEG, behavior, and cognition. All but two patients received hydrocortisone (initial dose of 5 mg/kg/day). The treatment was slowly tapered with a total duration of 21 months. There were 18 symptomatic and 26 cryptogenic cases. Mean age was 7 years and mean intelligence quotient/developmental quotient (IQ/DQ) was 65. Mean CSWS duration before corticosteroid treatment was years. Twenty patients had tried more than two antiepileptic drugs (AEDs) before steroids. Positive response to steroids was found during the first 3 months of treatment in 34 of 44 patients (%), with normalization of the EEG in 21 patients. Relapse occurred in 14 of them. Hence, 20 patients (%) were long-term responders after a single but prolonged trial of steroids, including all four cases of Landau-Kleffner syndrome. Positive response to steroids was highly significantly associated with higher IQ/DQ. Shorter CSWS duration, but not age, etiology, or previous AED trials, was associated with positive response to steroids. Early discontinuation of the treatment for side effects was encountered in seven patients. We conclude that corticosteroids are safe and efficient for treatment of epilepsy with CSWS. Poor responders are patients with very low IQ and long duration of CSWS.