VIRAL infections of the central nervous system (CNS) have attracted the attention of historians and physicians for millennia.1,2 With the exception of herpes simplex encephalitis,3,4 the specific causes of CNS syndromes of viral origin have been difficult and sometimes impossible to identify. With the advent of antiviral therapy, increased attention has focused on the prompt and specific diagnosis, pathogenesis, and treatment of viral infections of the CNS, especially herpes simplex encephalitis. This review will focus on the pathogenesis of such infections and the diagnostic approach to patients with presumed viral infections of the CNS. Although considerable emphasis will be placed… © 1990, Massachusetts Medical Society. All rights reserved.