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 a one 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 0.5 mg/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). 2.0 mg/kg/day (not to exceed 100 mg/day), 3.0 mg/kg/day (not to exceed 150 mg/day) and 4.0 mg/kg/day (not to exceed 200 mg/day). The planned sample size is 96 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 120 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 at 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 50 mg/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 2.0 mg/kg/day (not to exceed 100 mg/day). The fifth group of patients will receive 3.0 mg/kg/day (not to exceed 150 mg/day). The sixth group of patients will receive 4.0 mg/kg/day (not to exceed 200 mg/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.