Background: Epileptic encephalopathy with electrical status epilepticus in sleep (ESES) is a pediatric epilepsy syndrome with sleep induced epileptic discharges and acquired impairment in cognition, language and/or behavior. Despite the widespread use of high dose diazepam, there is limited research on its efficacy. Methods: Single-center, retrospective case-series of children presenting with cognitive/ language regression and ESES from 2014-2019. All children underwent baseline overnight EEG followed by diazepam (1mg/kg) administered per rectum, and continuation of 0.5 mg/kg of oral diazepam for 3 months. Follow up EEGs were performed following the first dose and after 6-9 weeks of treatment. Results: 23 children were included [male 14 (60%); mean age 7 years (4 -12)]. 10 children (45%) had symptomatic epilepsy (defined by abnormal MRI and/or genetic evaluation). Decrease in more than 25% of the spike activity was seen in 18 (78%). This effect was sustained in 11 children (47%) after 6 weeks. Only 6 (60%) children from the symptomatic group had EEG response, while 11 (91%) responded from the idiopathic group. 5 children (21%) had clinically significant cognitive/ language improvement. Conclusions: Treatment with diazepam reduces epileptiform activity in ESES in majority of children. Despite this reduction only minority of patients experience clinically significant cognitive improvement.