Objectives: Long-term outcome of Electrical Status Epilepticus during Sleep (ESES) is generally unfavourable but hard to predict in individual children. Longer duration of ESES and younger age at onset of ESES have been reported to be predictors of poor outcome, whereas any treatment response is associated with a relatively better prognosis. This study aimed at determining possible other early predictors of long- term outcome. Material and methods: We retrospectively studied a cohort of 35 children with ESES, treated at the University Medical Centre Groningen (UMCG) and the Epilepsy Clinic of SEIN, Zwolle. We examined possible early predictors including duration between onset of clinical symptoms and definite diagnosis of ESES (diagnostic delay), and spike-wave-index (SWI). To evaluate long-term outcome, school performance as well as cognitive, emotional and behavioural functioning was assessed with the parent- reported Brain Injury Alert - adapted for children with ESES. Results: Mean age at onset of epilepsy was 4.2 years; at onset of symptoms suggestive of ESES 6.5 years; and at diagnosis of ESES 7.6 years. SWI >85% was present in 40.0%, SWI 75-85% in 34.3%, and SWI 85% was overall associated with a less favourable outcome: categorized SWI differed significantly between children with and without a negative change in three parameters of the questionnaire: understanding language (p=0.013), depressive feelings (p=0.007), and indifference (p=0.049). Conclusions: In this retrospective study, both prolonged diagnostic delay and higher SWI are associated with unfavourable long-term outcome and might therefore be early predictors for cognitive, emotional and behavioural outcome in children with ESES.