Herpes simplex encephalitis (HSE) is associated with high mortality and morbidity, but these rates may be reduced by early administration of vidarabine. In six of 11 patients with the disease, the computed tomographic (CT) scan was of diagnostic value and showed low-density areas primarily in the temporal lobes; in only two of these six patients did the EEG point to the true diagnosis. Two of the six died. In the other five patients, the CT scan was normal or showed minor changes but the EEG was grossly abnormal. All five of these patients died. Information from CT scanning and the EEG should be combined for early diagnosis in HSE.