A Study on the Pharmacokinetics and Safety of Valcyte (Valganciclovir) in Pediatric Heart Transplant Recipients Less Than 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