ALTHOUGH the prognosis in herpes simplex encephalitis remains unclear in view of problems inherent in providing laboratory confirmation of diagnoses,1reports in current literature2-8indicate the disease is severe, often fatal, frequently leaving survivors with neurological sequelae. Until recently, treatment consisted largely of supportive care, with occasional administration of γ-globulin or hyperimmune plasma. During the past three years, several authors described beneficial results in herpes simplex encephalitis with systemic use of idoxuridine (5-iodo-2′-deoxyuridine).8-13However, no one has reported experience with systemic use of idoxuridine in the premature infant. Partridge and Millis14used intravenously administered idoxuridine in a combined, five-day course of intermittent and continuous infusion in a newborn term infant with systemic herpes simplex. The infant received a total dose of 580 mg/kg and showed clinical improvement, with no toxicity noted. The infant died later, probably from the effect of secondary bacterial infection. This report presents