The treatment of acute lymphoblastic leukemia (ALL) in children has become increasingly effective over the years. Currently, the cure rate is about 90%. This has become possible due to the use of combinations of appropriate chemotherapeutics in very high, often maximum doses. This is associated with the occurrence of so-called treatment-related mortality (TRM), the main cause of which is infections. The case report is presented of a 12-year-old patient who contracted a respiratory syncytial virus (RSV) infection during the consolidation phase of ALL treatment. The course of the infection was so severe that despite intensive antimicrobial treatment and a stay in an intensive care unit (ICU) the patient died. This indicates the absolute necessity of constant monitoring of a patient’s condition and consideration of early inclusion of antimicrobial prophylaxis, especially in patients at risk.