The courses of 15 brain biopsy-proven cases of herpes simplex virus encephalitis (HSVE) were followed for 6 to 67 months. Convulsive disorders were often temporary, but paralysis was permanent. Patients usually entered the hospital free of paralysis or coma, in a potentially reversible febrile confusional state. Later, paralysis and coma fixed subsequent courses. Mortality was 53.3 percent but, at follow-up, 93.3 percent (14 patients) were dead or living a vegetative existence at home or in institutions. If a definitive diagnosis of HSVE could be made at the time of hospital admission, the prognosis might be remarkably changed.