Neonatal herpes simplex virus (HSV) disease complicates approximately 1 in every 3,000 deliveries in the United States, resulting in an estimated 1, 500 cases annually in this country. HSV-1 and HSV-2 infections in the neonate can manifest as: disseminated disease; central nervous system (CNS) disease; or disease limited to the skin, eyes, and mouth (SEM disease). This study will evaluate the efficacy of long term suppressive therapy with oral acyclovir in infants with CNS disease, with or without evidence of dissemination to other organs (including the skin). It will determine if suppressive oral acyclovir therapy improves neurologic outcome in infants following HSV disease with CNS involvement and address the significance of a positive cerebral spinal fluid (CSF) polymerase chain reaction (PCR) result when all other CSF parameters either remain normal or show improvement. Comparisons will be made between groups with respect to post-randomization time to first positive CSF PCR result during the initial 12 months of life, and results will be correlated with clinical neurological assessments. It will determine if continuous administration of oral acyclovir suspension suppresses recurrent skin lesions in infants following HSV disease with CNS involvement, and it will confirm the safety of long-term administration of oral acyclovir therapy in a cohort of infants with HSV disease with CNS involvement. Finally, the effects of suppressive acyclovir therapy on issues of pharmacoeconomics and family infrastructure will be assessed and quantitated. Infants with CNS disease (with or without evidence of viral dissemination to other organs, such as the skin, liver, and lungs) will qualify for this study. Following a 21 day course of treatment with intravenous (IV) acyclovir, infants with CNS disease, with or without cutaneous involvement, will be randomized to either continuous oral acyclovir or placebo (CNS Sub-Study). Similarly, infants with disseminated disease with CNS involvement will be randomized to either continuous oral acyclovir or placebo (Disseminated with CNS Involvement Sub-Study). The subset of infants with CNS disease (with or without dissemination) who do not clear their acute infection in 21 days of IV acyclovir therapy will be eligible for enrollment in a Pilot Sub-Study. This group is expected to be of insufficient number to be able to obtain statistical significance for establishing efficacy. Per protocol amendment dated 19-Nov-1998, 66 subjects will be recruited into each sub-study. Subjects will begin oral drug therapy 8 hours after the final IV acyclovir dose and oral drug therapy will be administered for 6 months. Whole blood (1.0 cubic centimeter) will be obtained at study enrollment and at completion of IV antiviral therapy for HSV PCR analysis, per protocol amendment dated 4-May-1998. This amendment replaces the obtaining of serum for HSV PCR analysis. In the event that whole blood is not available, serum will be provided instead. All children will be followed at 6, 12, 24, 36, 48, and 60 months of age. Physical examination, hearing assessment, and retinal examination will be performed at each follow-up visit. Standardized neurological evaluations will be performed at 12, 24, 36, 48, and 60 months. The primary study endpoint will evaluate neurological impairment at 12 months of life. The secondary endpoints will evaluate post-randomization detection of HSV DNA in CSF by PCR at any time during the initial 12 months of life and 2 or fewer episodes post-randomization of cutaneous recurrence of HSV disease during the initial 12 months of life.