Interictal EEG abnormalities were evaluated in 111 epileptic patients in which the first seizure occurred after the age of 18 years. Standard day-time EEG tracings performed prior to and after antiepileptic treatment, and all-night recordings were investigated. Before treatment, waking EEG was normal in 54% of patients. The percentage of normal sleep EEG recordings was only 28.6%. Focal EEG abnormalities were found in 39% of day-time recordings and in 48.6% of sleep recordings, while the percentage of generalized epileptic discharges observed was respectively 7 and 22.8. When considering the category of epilepsy and the etiology, it appears that the primary and secondary generalization of epileptic discharges is reduced with age.