Fever and infection are serious complications of cancer treatment such as bone marrow transplant, especially when white blood cell counts are low. When the number of white blood cells is below 500, the person has a condition called neutropenia and has a high risk of developing an infection. At the first sign of fever, antibiotics are started. However, antibiotics do not kill fungus germs, and fungal infections may be difficult to treat. Thus, the prevention of fungal infections in this population is important. The only medicine approved for prevention of fungal infections is fluconazole, which prevents some but not all types of such infections. A new antifungal medication called FK463 works against more types of fungal infections than fluconazole does. This study will compare the effectiveness, safety, and tolerance of FK463 as compared with fluconazole.