Malignant neoplasms of the blood system in children are represented by highly aggressive variants, which at the stage of diagnosis and program chemotherapy can be complicated by the development of life-threatening conditions. Understanding the risk of possible complications allows you to effectively carry out preventive and therapeutic measures, to minimize adverse outcomes. This article presents modern approaches to the diagnosis and treatment of the most common life-threatening conditions in pediatric oncohematology: acute tumor lysis syndrome, typhlitis and neutropenic enterocolitis, superior vena cava syndrome, malignant airway compression syndrome, acute impairment of consciousness, spinal cord compression syndrome, thrombosis, methemoglobinemia.