To examine executive function, intelligence, visuospatial skills, language, memory, attention, reaction time, anxiety, depression, and emotional and behavioral traits most frequently associated with executive dysfunction in patients with juvenile myoclonic epilepsy (JME) compared with a sibling and a normal control group under video-electroencephalography (video-EEG) conditions. Twenty-two sibling pairs, one with JME, were compared with 44 controls matched for age, gender, and educational level. All participants were administered a comprehensive set of neuropsychological and questionnaire measures during and without video-EEG recording. The JME group differed significantly from controls in measures of phonemic and semantic verbal fluency. They scored significantly higher on the dysexecutive self-rating questionnaire, being more likely to report traits associated with executive dysfunction than both siblings and controls. Patients with JME reported significantly low mood than both controls and their siblings. Unaffected siblings differed significantly from controls on psychomotor speed, phonemic verbal fluency and were considered to exhibit traits associated with executive dysfunction by others. Qualitative inspection of data suggested a convincing trend for patients with JME and their siblings to perform worse than controls on most measures. This study supports the existence of a distinct neuropsychological profile among patients with JME and their siblings, which is likely to be genetically determined. The similarity of neuropsychological profiles between JME patients and their siblings is independent of antiepileptic drug effects or subclinical EEG activity. The significant differences between the sibling and controls suggests that there is a neurocognitive endophenotype for JME.