This paper reports on 294 patients with presumptive symptoms of epilepsy but no noteworthy, or only suspicious, EEG findings. With recording during sleep after 24–27 h of sleep deprivation, 138 persons showed definite epilepsy potentials. The remainder of the group showed no noteworthy findings. Nevertheless, on the basis of clinical observations and the course of the patient’s illness, epilepsy was diagnosed in 27 patients of this second group. Neurophysiological aspects of the electroence phalographic provocation method are discussed.