The authors recorded generalized and focal periodic discharges in 49 EEG tracings from 27 patients aged 5 to 75 years. According to the shape of the waves and their complexes generalized periodic discharges were formed by slow waves, sharp waves or spikes, triphasic waves and periodic activity of the "suppression burst" type. Focal periodic discharges had the shaps of slow waves and so-called periodic lateralized epileptiform discharges. Six patients suffered from Creutzfeldt-Jacob's disease and from herpetic encephalitis. Four had an acute cerebrovascular attack. Three suffered from subacute sclerotizing panencephalitis. Other conditions found included metabolic brain damage, post-traumatic purulent meningitis, anoxic encephalopathy and partial motor convulsions. Different types of periodic discharges were found in the same disease and even in the same patient and conversely one type of periodic discharges is found in different diseases. The authors assume that periodic discharges are not specific, have a different aetiology but obviously a common pathophysiological mechanism. From the EEG alone conclusions on the aetiology can be drawn only with a limited probability. It is necessary to use a comprehensive approach in their evaluation, taking account age, the clinical picture and development as well as results of other examinations. The 70.3% mortality in the group provides evidence that periodic discharges are a serious finding as regards the prognosis of the disease.