High-dose (HD) methotrexate (MTX) is a critical component of treatment for hematologic malignancies in children and young adults. Therapeutic drug monitoring is necessary due to substantial interindividual variation in MTX clearance. Common function-altering polymorphisms in SLCO1B1 (encodes OATP1B1, which transports MTX) may contribute to clearance variability. We performed pharmacokinetic modeling using data for 106 children and young adults treated with HD MTX for hematologic malignancies; of 396 total courses of HD MTX, 360 consisted of 5 g/m 2 over 24 hours. We evaluated the contribution of clinical covariates and SLCO1B1 genotype (388A>G and 521T>C) to MTX clearance variability. Of the clinical covariates studied, patient weight improved the pharmacokinetic model most significantly (P < 0.001). The addition of the SLCO1B1 variants individually further improved the model (P < 0.05 for each). An interaction between these variants was suggested when both were included (P = 0.017). SLCO1B1 genotype should be considered in efforts to personalize HD MTX dosing. Methotrexate (MTX) is an antifolate chemotherapy agent. High-dose (HD) MTX regimens (> 500 mg/m 2 per dose), integral to the treatment of children and young adults with acute lymphoblastic leukemia (ALL) and other malignancies, are administered intravenously and require significant inpatient supportive care to reduce risk for toxicities. The addition of HD MTX to therapy for high-risk B-cell ALL significantly improved survival for high-risk B-cell ALL 1 and reduced central nervous system relapse in lymphoblastic lymphoma. 2,3 Although highly effective, HD MTX has a narrow therapeutic window with potentially severe toxicities (e.g., myelosuppression, mucositis, hepatotoxicity, nephrotoxicity, and neurotoxicity). 4 Drug level monitoring is required to reduce risk for toxicities. Greater than 90% of children diagnosed with cancer in the United States are treated per Children's Oncology Group (COG) protocols, on which supportive care measures are adjusted for supratherapeutic MTX levels but no changes are made for low MTX levels, suggesting rapid clearance. 4 Drug clearance following HD