This open label study will assess the pharmacokinetics and the safety and tolerability of Valcyte (valganciclovir) powder for oral solution in neonatal and infant heart transplant patients < 4 months of age.

Inclusion Criteria:

• Males and females, 0 to < 4 months (< 125 days) of age at the time of the last PK assessment in this study
• Parent or guardian of the patient is willing and able to give written informed consent
• Patient has received a first heart transplant
• Patient is at risk of developing cytomegalovirus (CMV) disease and is being treated with i.v. ganciclovir or oral valganciclovir for prevention of CMV
• Adequate hematological and renal function
• Able to tolerate oral medication (any appropriate form of tube feeding is acceptable)

Exclusion Criteria:

• Allergic or significant adverse reaction to acyclovir, valacyclovir or ganciclovir in the past
• Severe, uncontrolled, clinically abnormal diarrhea
• Liver enzyme elevation of more than five times the upper limit of normal for AST (SGOT) or ALT (SGPT)
• Patient requires use of any protocol prohibited concomitant medications
• Patient has previously participated in this clinical trial