Of 901 patients investigated 167 showed a focal disturbance in the EEG before and/or after 24-hours sleep deprivation. 459 patients suffered from epileptic seizures, the remainder had non-epileptic attacks or an organic cerebral disease without seizures. Of the 167 focal EEG changes 21.6% were seen only following sleep deprivation. In 9.6% focal changes present before could not be observed anymore following sleep deprivation. There was no significant difference between the three groups of patients mentioned above. The 24-hours sleep deprivation thus represents an activation procedure which may enhance diagnostic information relating to the question of focal EEG disturbances not only in epileptic patients. In the group of patients with epileptic seizures the activation or suppression respectively of the focal disturbance showed some dependence from seizure type and frequency as well as from the type of the focal change (spikes or slow waves). In our material the differences were however not statistically significant. More information could be gained from the waking EEG and from stage 1 than from sleeping stages 2 to 4 according to Rechtschaffen and Kales. Trigger functions could be attributed especially to the transitional phases from wakefulness to sleep and from stage 1 to stage 2 and vice versa particularly associated with arousal reactions. In our short sleep records REM stages could be rarely observed.