• Six patients with herpes simplex virus (HSV) encephalitis underwent diagnostic and clinical evaluation. The HSV or its antigenic material was found in three brain biopsy specimens. In the remaining three cases, the diagnosis was supported by detection of HSV antibodies in the CSF. Cell count and total protein concentration in the CSF reached a maximum level at three weeks and two months, respectively. The IgG index and HSV antibody level in the CSF often remained constant after reaching maximal values. In three patients, a transient low serum sodium level was observed. Characteristic EEG changes were present five to 11 days after the onset of symptoms. Computerized tomographic scanning revealed a temporal low-density lesion. Three patients became deeply comatose and had respiratory failure. The patient without vidarabine therapy and one of the five patients treated with vidarabine died.