Many published studies report an increased incidence of epileptiform EEG activity following sleep deprivation in persons with epilepsy in whom a previous routine EEG was normal or inconclusive. Few such studies, however, permit a clear distinction to be made between the effects of sleep deprivation per se, sleep induction following deprivation, or simply repeated EEG recording. Sixty-nine patients have been investigated in whom a routine waking record, a secobarbital-induced sleep recording and an EEG following 24 h sleep deprivation were obtained in random order irrespective of whether or not the initial EEG contained epileptiform activity. For each record the incidence of epileptiform activity if any, was quantified in terms of discharges per minute for wake, drowsiness and each sleep stage. The findings confirmed the marked activating effect of light sleep on the EEG but there was no evidence of an overall increase in discharge rate after sleep deprivation either in the waking state or in the various sleep stages when these were compared with secobarbital-induced or spontaneous sleep. There was a significant increase of generalized discharges in the waking state only, after sleep deprivation, but a decrease in incidence of focal epileptiform activity. It is concluded that although specific indications may exist for the use of sleep deprivation, as a general method of EEG activation in epilepsy it has no advantages over barbiturate-induced sleep to offset the greater inconvenience to patients.