The objective of this study is to evaluate the safety, tolerance, and pharmacokinetics of FK463, a novel echinocandin (cell wall-active antifungal lipopeptide), as early empirical therapy for prevention of fungal infections in immunocompromised children. The study is designed as a multicenter open label, sequential dose escalation study of intravenous FK463. Intravenous FK463 will be administered daily as an hour infusion to patients with new onset of fever and neutropenia (absolute neutrophil count less than or equal to 500/mm3) who will be initiated onto broad spectrum empirical antibacterial therapy. The patient population consists of children ages 2 to 17 years of age; two age cohorts will be studied (2-12, 13-17). Dosage levels will be 0.5mg/kg/day (not to exceed 25 mg/day), 1.0 mg/kg/day (not to exceed 50 mg/day), 1.5 mg/kg/day (not to exceed 75 mg/day) and 2.0mg/kg/day (not to exceed 100mg/day). The planned sample size is 64 patients (a maximum of two replacement patients may be added to a given dose level and age cohort, for a total of no more than 10 patients per dose level and age cohort. The study will enroll no more than 80 patients). At each dosage level, a total of 8 patients will be enrolled into each age cohort (2-12, 13-17); a total of 16 patients will be enrolled into each dosage level. The first group of patients will receive FK463 at 0.5 mg/kg/day (not to exceed 25 mg/day). The second group of patients will receive 1.0 mg/kg/day (not to exceed 50mg/day). The third group of patients will receive 1.5 mg/kg/day (not to exceed 75 mg/day). The fourth group of patients will receive 2.0mg/kg/day (not to exceed 100mg/day). Study drug will continue until recovery from neutropenia (ANC post nadir greater than or equal 250/mm3) or until the initiation of conventional deoxycholate amphotericin B or a lipid formulation of amphotericin B for empirical antifungal therapy or for proven fungal infection. Patients may receive FK463 for a maximum duration of 14 days. For any patient who meets institutional criteria to start standard empirical antifungal therapy with conventional deoxycholate amphotericin B or a lipid formulation of amphotericin B (greater than 96 hours on study drug) or who has a proven breakthrough fungal infection, FK463 will be discontinued and conventional deoxycholate amphotericin B or a lipid formulation of amphotericin B will be initiated.