Herpes simplex is the most common cause of sporadic viral encephalitis. The recent development of specific antiviral chemotherapeutic agents offers new optimism for patients with this disorder if therapy is begun on or before the fifth day of the disease. Eight patients with herpes simplex encephalitis were studied by CT, and a characteristic but not pathognomonic pattern was observed. In each case a low density lesion was noted in the medial portion of the temporal lobe with extension into the Island of Reil. Sparing of the lenticular nucleus was observed in all cases. Mass effect and streaky linear enhancement after contrast administration was also seen. Unfortunately, the findings may be subtle or absent before the fifth day of disease, and thus CT scans must be examined with a high index of suspicion if the correct diagnosis is to be made at a time when therapy may prove useful. Hemorrhagic areas are rarely observed on CT in this disorder despite the frequent occurrence on pathologic studies. The full extent of involvement may not be appreciated on scans obtained during the first 10 days of the disease.