Methotrexate (MTX) is the chemotherapeutic fdr which the serum levels can be detected. If the MTX level is detected in time, high toxicity risk can be decreased. In this study, intermediate doses of MTX (1 g/m(2)) infusions are administered to B-cell non-Hodgkin lymphoma patients between 3 and 13 years old. The toxicity of MTX in accordance with serum levels and the toxicity of other combined drugs are investigated. Blood samples were collected consecutively, and MTX levels were detected by highperformance liquid chromatography. When hematological, gastrointestinal, and renal toxicity scores were compared with the 24-h serum levels of MTX, they showed a significant positive correlation. Hematological toxicity scores increased by Ifosfamide, Etoposide, and Cytarabine combined with MTX without altering the serum levels. Antibiotic combination with MTX has no effect on the toxicity scores. In conclusion, if MTX is combined with other myelosuppressive, hepatotoxic, and nephrotoxic drugs, the measurement of MTX serum levels alone is not a sufficient parameter to show the toxicity. (C) 1999 Elsevier Science Inc. All rights reserved.