Inclusion Criteria

Inclusion criteria:

- Age: Patients must be > / = 2 and < / = 18 years old.

- Life Expectancy: Patients must have a life expectancy of > 8 weeks

- Subjects must have a diagnosed malignancy, and must currently be receiving
consolidation or maintenance chemotherapy

- Hepatic/Renal Function: Patients must have adequate hepatic function (bilirubin < / =
1.5 mg/dl: SGPT < 3x normal) and adequate renal function (creatinine < / = 1 mg/dl:
BUN < 20 )

- Medication Tolerance: Patients must be able to swallow pills or tolerate a suspension
of the medication

- Fluid Tolerance: Children must be able to retain liquids at the time of enrollment

- Informed Consent: Written informed consent will be obtained from all patients and/or
their parents prior to enrollment

- Pregnancy: Women of childbearing age must have a negative serum pregnancy test at the
time of study entry

Exclusion criteria:

- More than one prior chemotherapy regimen.

- Patients with uncontrolled infections.

- Subjects with known history of adverse reaction to acyclovir in the past.

- Patients with concurrent infections requiring treatment with valacyclovir or
acyclovir.