The study of juvenile myoclonic epilepsy is important in that: it is common and heterogeneous; the etiology is unknown; and patients report broad cognitive problems. We utilized a broad battery of neuropsychometric tests to assess the following: intellectual function, memory, language and naming, executive function, the impact of epilepsy, and antiepilepsy drug side effects. Sixty people with drug-refractory JME were interviewed, and performance was profoundly impaired across the range of tests. Impairments included the following: full-scale IQ (89, p<0.001); processing speed (86, p<0.001); visual memory (immediate and delayed) more affected than verbal memory; verbal fluency and inhibition (p<0.001); and self-reported drug side effects (p<0.001). Eighty-three percent of patients exhibited frank executive dysfunction, which was moderate to severe in 66%. Regression modeling confirmed that an early age at onset and the need for polytherapy were associated with poorer cognitive outcomes. This study confirms previous reports of executive dysfunction in a larger cohort and with greater statistical rigor. We also identified a high prevalence of neurotoxicity symptoms such as fatigue and poorer functioning across intellectual and memory tests than had previously been reported.