We evaluated adenine arabinoside (vidarabine) for treatment of herpes simplex encephalitis in a placebo-controlled study. In 28 cases proved by isolation of Type 1 virus from brain biopsy, treatment reduced mortality from 70 to 28 per cent (P = 0.03), and over 50 per cent of treated survivors had no or only moderately debilitating neurologic sequelae. This improvement was achieved without evidence of acute drug toxicity. Thus, adenine arabinoside has a good therapeutic index (efficacy/toxicity) for the treatment of Type 1 herpes simplex encephalitis. However, the drug must be given early in the course of infection before the advent of coma to have a beneficial effect. Moreover, it should be coupled with brain biopsy for specific diagnosis to avoid unnecessary treatment of nonresponsive encephalitides that can mimic herpes simplex.