Abstract: OBJECTIVE To investigate the relationship between the plasma concentration and adverse reactions of high dose methotrexate (MTX) used in the treatment of pediatric acute lymphoblastic leukemia. METHODS The concentration of MTX in plasma samples of 72 patients were measured by a flurorescence polarization immunoassay, which were analyzed combined with the clinical manifestions by Chi-square test. RESULTS When the MTX C_{24 h}≥40 μmol·L^-1, the incidence rates of adverse reaction were higher than those of MTX C_{24 h}<40 μmol·L^-1(P<0.05). When the MTX C_{48 h}≥1.0 μmol·L^-1, the incidence rates of adverse reaction were higher than those of MTX C_{48 h}<1.0 μmol·L^-1(P<0.01). CONCLUSION The incidence rates and the severity of adverse reactions of high dose MTX were ralated directly with the level and the persistence time of the plasma concentration.