Objective:To investigate the association between MTHFR C677T polymorphism and toxicities of methotrexate-induced toxicities of childhood acute lymphoblastic leukemia. Methods:A total of 98 children with acute lymphoblastic leukemia were selected. And PCR-RFLP was used to determine the genotypes of MTHFR C677T polymorphism. The toxicity response of patients received high-dose methotrexate chemotherapy was observed. Results:Compared with the wild-type CC,the carries of the variant genotypes (CT and TT) had higher methotrexate plasma concentration. Conclusion:MTHFR C677T polymorphism is associated with the toxicities of methotrexate-induced toxicities of childhood acute lymphoblastic leukemia,mainly gastrointestinal reaction and bone marrow suppression.