The most common adult primary brain tumor is glioblastoma multiforme (GBM). Current treatment is surgical resection, adjuvant radiation and chemotherapy, which can extend the median survival 20-36 weeks (Mansky et al. Central nervous system tumors. In Abraham J, Allegra CJ, Gulley J, eds. Bethesda Handbook of clinical oncology, 2nd edn. Philadelphia, Pennsylvania: Lippincott Williams and Wilkins, 2000: 440-2; Knox S. Intracranial tumors. In Pillot G, Chantler M, Magiera H, Peles S, et al., eds. The Washington Manual Hematology and Oncology Subspecialty Consult. Philadelphia, Pennsylvania: Lippincott Williams and Wilkins, 2004: 204-6.). But treatment efficacy is limited, mandating the exploration of more effective treatments. We report on a patient with GBM treated as per a clinical protocol with high-dose methotrexate (12 g/m(2)), who expired within hours after the initiation of treatment secondary to transtentorial herniation. Although it is not completely clear what caused the patient's herniation, we think that high-dose methotrexate therapy may have played a crucial role. We suggest that high-dose methotrexate should be used cautiously in patients with GBM.