Infusions with high-dose methotrexate 5 g/m2 or 8 g/m2 are used to treat children with acute lymphoblastic leukemia (ALL), non-Hodgkin's lymphoma, medulloblastoma and ependymoma. Methotrexate is primarily excreted unchanged by the kidney where it can course acute kidney damage resulting in prolonged time of excretion of the drug. Our main hypothesis is that 12 hours of intravenous hydration before the methotrexate infusion is more efficacious in preventing methotrexate induced kidney damage compared to four hours of hydration.