Infusions with high-dose methotrexate 5 g/m2 or 8 g/m2 are according to the protocol of the Nordic Association for Pediatric Hematology and Oncology 2000 (NOPHO-2000) used to treat children with acute lymphoblastic leukemia (ALL). Treatment with methotrexate 5 g/m2 is also used to treat children with non-Hodgkin lymphoma, medulloblastoma and ependymoma. Methotrexate is primarily excreted unchanged by the kidney where it can course acute nephrotoxicity resulting in prolonged elimination time of the drug. Data from a 10 years retrospective investigation at our pediatric unit show, that in spite of urine alkalinization and intensive hydration the elimination of methotrexate is prolonged in 20-50% of the infusions. The long exposure of a high serum methotrexate concentration is associated with an increased frequency of mucositis and bone marrow suppression. Further more the need of rescue with folic acid is problematic, because it is possibly that it can result in a higher risk of relapse of ALL (Leukemia 2006; Skarby TV). Most ALL protocols prescribe prehydration of 2-6 hours before initiation of the methotrexate infusion and it has never been investigated in a randomized controlled trail if a longer time of prehydration can prevent nephrotoxicity and reduce the risk of prolonged elimination. In our pediatric unit the prehydration is given with a rate of 150 ml/m2/hour with a solution of 5% glucose with 40 mmol sodium bicarbonate/L and 20 mmol potassium chloride/L.