Electroencephalograms (73) of 11 patients with confirmed herpes simplex encephalitis were recorded. Based on serial recordings it was seen that the repetitive complexes could be recorded at any time of the illness; they were associated with a fatal outcome. The repetitive complexes were absent in all patients who survived. Long-term follow-up of the surviving patients shows spike discharges, which were transitory and not associated with clinical signs in three of them, with a return to normal rhythmic activity. It is proposed that the EEG may be used as a prognostic sign in herpes simplex encephalitis.