We describe a 64-year-old woman with biphenotypic leukemia involving the meninges who received 2 doses of intrathecal methotrexate. Soon after treatment, the patient developed postural rigidity and a marked decline in mental status. The patient died of respiratory failure 1 month after methotrexate treatment was initiated. At autopsy, the brain was grossly normal. Routine microscopy showed no evidence of leukemic infiltrates or necrotizing lesions. However, when stained with beta-amyloid precursor protein, multifocal axonal injury was evident in the brain, spinal cord, and nerve roots. Our findings show that immunohistochemical staining for beta-amyloid precursor protein can effectively demonstrate axonal injury associated with methotrexate neurotoxicity, even when conventional staining procedures are negative. This technique may therefore reveal a possible pathologic substrate for some of the neurological complications seen in patients with methotrexate neurotoxicity.