Eight hundred patients will be enrolled in this study. They will be randomly assigned to receive either fluconazole or FK463. Before the medicine is begun, a physical exam as well as a blood sample, mouth swab, urine sample, and chest x-ray will be done. The fluconazole or FK463 will be administered once a day for one hour into the bloodstream through a catheter in the vein. Blood tests will be taken twice a week. Cultures from the blood, mouth, and urine will be taken throughout the study. X-rays and CT scans will only be taken if a fungal infection is suspected. If fever develops, blood will be drawn to check for fungi. If fever and neutropenia continue for more than 4 days, FK463 or fluconazole will be stopped and a standard medication called amphotericin B will be started.