In this study, we describe the disease evolution and adult phenotype of KCNQ2 encephalopathy through retrospective study of 13 patients (18-45 years of age). We show that seizure frequency declines over the years and that most patients are seizure-free in adulthood. Longer seizure-free periods followed by seizure recurrence are however common during childhood and adolescence. Tonic seizures are the dominant seizure type during infancy, and tonic-clonic and focal impaired awareness seizures later in life. The relative importance of non-epileptic co-morbidities increases with age, as most adult patients have severe intellectual disability, and motor, language and behavioural problems become an issue of continuous concern. (https://jmg.bmj.com/content/early/2021/04/01/jmedgenet-2020-107449 )