In a series of six cases of encephalitis caused by herpes simplex virus, type 1, the diagnostic and prognostic value of the electroencephalogram (EEG) was investigated. Special interest was focused on the time relationship between the appearance of abnormal EEG findings, the initial clinical symptoms and the changes in cranial computed tomography (CT). The characteristic periodic EEG pattern can be demonstrated within 2 days of disease, before typical structural changes appear in CT. A unilateral periodic pattern may be associated with a good prognosis, whereas all patients with bilateral changes died in spite of specific antiviral therapy.