This study will assess the safety and efficacy, and the pharmacokinetics of aciclovir and valaciclovir in children with chickenpox following oral administration of valaciclovir, for the purpose of seeking approval of valaciclovir HCl for the treatment of chickenpox.

Inclusion criteria:

• Subjects aged 1 year to less than 12 years with a clinical diagnosis of chickenpox within 48 hours of the onset of the rash.

Exclusion criteria:

• History of hypersensitivity reactions.
• Impaired hepatic or renal function.
• Gastrointestinal dysfunction.
• Serious underlying disease.
• Weigh over 40kg.
• Vaccinated for chickenpox.