A total of 208 patients underwent brain biopsy for presumptive herpes simplex encephalitis and were randomized to receive either vidarabine, vira-A, at 15 mg/kg/day, or acyclovir, at 30 mg/kg/day for ten days. 69 patients (33%) had biopsy-proven disease; 37 received vira-A and 32 acyclovir. With the exception of age, patient populations were balanced for demographic characteristics. Overall survival for acyclovir recipients was 72% compared with 46% for vira-A-treated patients 18 months after therapy (p=0.008). After adjustment for differences of age between treatment populations by multivariant regression analyses, acyclovir treatment remained superior to vidarabine therapy (p=0.041). Mortality varied according to the level of consciousness at the onset of therapy. For lethargic, semicomatose and comatose patients, mortality was 42%, 46%, and 67%, respectively, for the vira-A-treated patients and 0%, 25% and 25%, respectively, for acyclovir-treated patients. Six months post-therapy morbidity assessments revealed five (14%) vira-A versus 12 (38%) acyclovir recipients who had returned to normal function, while eight (22%) and three (9%), respectively, had moderate debility. Outcome differences were significant (p=0.02; Wilcoxon, 2-sample test) using an adapted scoring system. Age and Glasgow coma scale > 10 predicted the best outcome following acyclovir treatment. Disoriented patients who flex and respond by eye to pain had no mortality and 50% returned to normal. These data indicate that acyclovir is the treatment of choice for biopsy-proven herpes simplex encephalitis. © 1987 MMV Medizin Verlag GmbH München.