Background. Valacyclovir provides enhanced acyclovir bioavailability in adults, but limited data are available in children. Methods. Children 1 month through 5 years of age with or at risk for herpesvirus infection received a single 25 mg/kg dose of extemporaneously compounded valacyclovir oral suspension (n = 57), whereas children 1 through 11 years of age received 10 mg/kg valacyclovir oral suspension twice daily for 3-5 days (herpes simplex virus infection) (n = 28) or 20 mg/kg 3 times daily for 5 days (varicella-zoster virus infection) (n = 27). Blood samples for pharmacokinetic analysis were collected during the 6 h after the first dose. Safety was monitored throughout the studies. Results. Dose proportionality in the maximum observed concentration (Q max) of acyclovir and the area under the concentration-time curve from time zero extrapolated to infinity (AUC 0-∞) existed across the 10 to 20 mg/kg valacyclovir dose range. For children 2 through 5 years of age, an increase in dose from 20 to 25 mg/kg resulted in near doubling of the C max and AUC 0-∞. Among infants 1 through 2 months of age receiving 25 mg/kg, the mean AUC 0-∞ and C max were higher (∼60% and 30%, respectively) than those among older infants and children receiving the same dose. Valacyclovir oral suspension was well tolerated. No clinically significant trends were noted in clinical chemical, hematologic, or urinalysis values from screening to follow-up. Conclusions. Among children 3 months through 11 years of age, the 20 mg/kg dose of this formulation of valacyclovir oral suspension produces favorable acyclovir blood concentrations and is well tolerated. A dosing recommendation cannot be made for infants <3 months of age because of decreased clearance in this age group. © 2009 by the Infectious Diseases Society of America. All rights reserved.