An open study of vidarabine (adenine arabinoside) therapy was performed to verify the mortality from neonatal herpes simplex virus infection and to define further long-term morbidity. A total of 39 babies not previously reported were treated with either 15 mg/kg/d (16 newborns) or 30 mg/kg/d (23 newborns) of vidarabine administered intravenously for ten to 14 days. Outcome was compared with that from 56 newborns evaluated in a prior trial. Irrespective of the dose of medication, therapy decreased the mortality in babies with disseminated and CNS disease to 40%. The extent of organ involvement and, in particular, pulmonary herpes simplex infection were predictive of mortality (P = .001, for both). For these babies, 32% achieved normal developmental milestones 2 years after therapy. Disease localized to the skin, eye, and/or mouth was not associated with death. However, neurologic impairment occurred in 12% of this treated group of newborns. These findings underscore the value of vidarabine therapy of neonatal herpes simplex virus infection. However, an increase in dosage did not appear to result in significant improvement in either mortality or morbidity. Further improvement in the mode of therapy and the utilization of more potent antiviral drugs are currently being tested.