A total of 432 patients underwent brain biopsy for presumptive herpes simplex encephalitis. Three patient groups were identified. The first group, 195 patients (45%), had herpes simplex encephalitis confirmed by the isolation of herpes simplex virus from brain tissue at biopsy (193 patients) or autopsy (2 patients). The second group, 95 patients (22%), had diseases that were identified but that were not caused by herpes simplex virus. Three subgroups were recognized: (1) 38 patients (9%) with treatable disease, (2) 40 patients (9%) with nontreatable but diagnosed viral infection, and (3) 17 patients (4%) with identified diseases neither of viral etiology nor treatable. The third group, 142 patients (33%), remained without a diagnosis. Clinical presentation of patients in the second group was similar to that of those with herpes simplex encephalitis and those without a diagnosis. Patients in the subgroup with nontreatable but diagnosed viral infections had the greatest likelihood of returning to normal. © 1989, American Medical Association. All rights reserved.