The EEGs of 21 newborns with herpes simplex encephalitis were analysed. The diagnosis was based on the demonstration of herpes simplex infection in association with neurological symptoms, other etiological factors being excluded. Sixteen of 21 babies (76%) showed in their EEGs electrical seizures, either focal or unilateral. These paroxysms consisted of sharp waves or slow waves repeating at pseudo-periodic intervals, usually of 0.5-2 sec. Individual paroxysms had a duration of 10-20 sec in 6 babies and 1-2 min in 10 patients. During the same period, 20 other babies displayed the same EEG finding. Of these, 11 had encephalitis of unknown etiology. In our series there appeared to exist a correlation between both the duration of the electrical seizures and the degree of EEG background abnormality and the clinical outcome. Babies with markedly abnormal background and long-lasting paroxysms tended to have a poor prognosis. It is concluded that in newborns with clinical signs of encephalitis who show in their EEGs paroxysms of the type described, the possibility of herpes simplex encephalitis should be considered.