This is a randomized, double-blind, multicenter, prospective, comparative study of fluconazole versus voriconazole for the prevention of fungal infections in allogeneic hematopoietic transplant recipients and cord blood recipients in children under the age of 12. Prior to the start of the pre-transplant conditioning regimen, participants will give written informed consent and be screened for eligibility. Participants who meet all entry criteria will be assigned randomly to voriconazole or fluconazole within 72 hours of Day 0. Participants will begin the study drug on Day 0 (after completion of the conditioning regimen). Day 0 is defined as the day infusion of the stem cell product is completed. The study drug will be continued until Day 100 following transplant or until one or more criteria for early withdrawal are met. Continuation of the study drug beyond Day 100 is permitted for participants who meet specific criteria. The development of any fungal infection during prophylaxis will be classified according to the definitions listed in the protocol.