24 infants consecutively treated with acyclovir or vidarabine for neonatal herpes simplex virus (HSV) encephalitis were followed up for 6 months to 3 years to assess neurological and developmental outcome. 15 patients had HSV-2 and 9 had HSV-1 encephalitis. Infants with HSV-2 encephalitis presented with a higher frequency of seizures, greater pleocytosis and protein concentrations in the cerebrospinal fluid, and more frequent evidence of structural damage on computerised tomographic scans of the brain than did those with HSV-1 encephalitis. 1 patient died. All 9 HSV-1 patients were normal at follow-up (mean 19.4 months) compared with only 4 (23%) of the 14 surviving HSV-2 infected infants (p = 0.003). Among infants with HSV-2 encephalitis, 50% became microcephalic; 57% had seizure disorders; 64% had ophthalmological defects; 64% had cerebral palsy; and 57% had mental retardation. Infants with neonatal HSV-1 encephalitis treated with systemic antiviral chemotherapy have excellent neurological outcomes; the neurological morbidity of those with HSV-2 encephalitis is still high.