Herpes simplex virus lesions recur in 8-30% of infants who receive a course of parenteral antiviral therapy for an initial infection. Long-term acyclovir is used by some clinicians to prevent recurrent Herpes simplex disease. We describe nine infants who were treated with doses of oral acyclovir which were chosen to achieve 2-h post-plasma concentrations of > or = 2 micrograms/ml. Eight infants had Herpes simplex encephalitis and one had multiple recurrences of dermal and ocular disease. The target plasma concentration was chosen in order to attain acyclovir cerebrospinal fluid distribution (< or = 50% plasma) for an estimated ID30 of Herpes simplex II strains of 0.1-0.5 microgram/ml. One of nine patients failed to achieve the target plasma acyclovir concentration. One of nine patients developed symptomatic recurrence of the central nervous system disease and none of the remaining eight patients experienced recognized dermal or neurologic recurrence of Herpes simplex disease. Renal and neurologic status were routinely monitored and no signs of acyclovir toxicity were observed. Plasma concentration of acyclovir > or = 2 micrograms/ml may be achieved with average oral doses of 1340 mg/m2/dose (1000-1740 mg/m2/dose) given at 12-h intervals.