Attention deficit hyperactivity disorder is a syndrome of unknown etiology that affects 3% to 7% of the population. Although no objective test exists to support the diagnosis, EEG discharges related to the neuropsychiatric abnormalities have a high incidence in attention deficit hyperactivity disorder. The aim of this study was to elucidate the quantitative and qualitative characteristics of EEG abnormality in attention deficit hyperactivity disorder. Nonrapid eye movement sleep EEGs were evaluated in 134 patients with attention deficit hyperactivity disorder in terms of age, EEG timing, spike and wave index, foci, and seizures. All patients had spike and wave paroxysms that changed with age at and time to first EEG. Only half of the patients had seizures and 46 patients had neither seizures nor foci. Thirty-eight patients had the EEG findings of benign focal epilepsy of childhood. Our data support the view that spike and wave activity evolves in time and that EEG discharges are related to neuropsychiatric symptoms even when a diagnosable seizure disorder is absent.