The objective of this study is to determine the efficacy and safety of FK463 versus fluconazole in preventing fungal infections in patients undergoing an autologous (for hematologic malignancies) or allogeneic hematopoietic stem cell transplant. This is a Phase III, multicenter, randomized, double-blind study in patients six months of age and older. Study drug, either FK463 at 50 mg/day (1.0 mg/kg/day in patients weighing less than 50 kg) or fluconazole at 400 mg/day (8 mg/kg/day in patients weighing less than 50 kg), will be administered intravenously once daily in a blinded manner. Study drug will continue until neutrophil recovery (defined as a post nadir absolute neutrophil count (ANC) of greater than or equal to 500/mm3). Patients who develop a proven, probable, or suspected (requires empirical antifungal therapy) fungal infection will be discontinued from the prophylactic regimen. The maximum time the patient may receive study drug is 42 days post transplant.