We present a 10-year-old male patient with a diagnosis of relapsed acute myeloid leukemia (AML), presenting with high-risk febrile neutropenia (HRFN), after a cycle of intensive chemotherapy, evolving with an invasive fungal infection demonstrated by histopathology. Treatment with intravenous voriconazole was started, with erratic plasmatic levels, which require successive dose adjustments which also occurred with oral administration. Finally, he had a favorable response to treatment, despite of the dosing difficulties to reach therapeutic levels. Active search as well as preemptive antifungal therapy, together with plasmatic level monitorization of voriconazole allowed a prompt recovery and improved the patient prognosis.