Methotrexate (MTX) is a widely used anti-tumor drug, folic acid antagonists. The effectiveness of high doses (more than 1 g/m2) of MTX in a monotherapy or in a combination with other chemotherapeutic drugs has been proven in a treatment of a large spectrum of oncological diseases in children such as osteosarcoma, tumors of the central nervous system, lymphomas, acute lymphoblastic leukemia. The use of high doses of MTX is often fraughted with development of life-threatening complications such as MTX-induced acute kidney injury, neurotoxicity, myelosuppression. Currently developed recommendations for therapeutic monitoring and supportive care, including hyper-hydration, urine alkalization and leucovorin therapy, allow to reduce toxicity of MTX, but in some patients it is not always possible to prevent the development of complications. Cohort studies in order to identify risk factors for toxicity after high doses of MTX in children with different forms of cancer are limited, as well as comparing various types of supportive care regimens, including, infusion therapy. Studying of development mechanisms of MTX toxicity depending on background conditions will allow to improve existing recommendations and develop new and most effective standards of supportive therapy with respect to the individual specifics of patients.