Purpose: To identify and define clinical and behavioral features of patients with adult-onset idiopathic generalized epilepsy (IGE). Methods: We reviewed the charts of 313 IGE patients at the NYU Comprehensive Epilepsy Center over the past 5 years to identify patients with adult onset (18 years old or older). We excluded patients with childhood or adolescent symptoms that suggested absence, myoclonic, or tonic-clonic seizures, as well as those with a history of significant head injury or other known causes of localization-related epilepsy. Results: Forty-two (13.4%) patients had a clear onset of IGE in adulthood; average age of onset was early 20s (mean, 23.8 years range, 18-55 years). Twenty-one patients had adult myoclonic epilepsy (AME, 50%), and three had generalized tonic-clonic seizures on awakening (GTCS-A, 7%). More than two thirds (n = 30) are well controlled with current antiepileptic drugs (AEDs), and almost 90% are currently employed (n = 37). One third were diagnosed and treated for mental disorders, including depression (n = 12), anxiety (n = 7), obsessive-compulsive personality disorder (n = 2), and postictal psychosis (n = 1). Conclusions: Adult-onset IGE is associated with a good prognosis. An association may exist between psychological disorders, psychotropic medication, and level of seizure control in adults with IGE.