Myelopathy is an uncommon complication of radiotherapy, particularly in the pediatric age group. A 5-year-old girl with acute lymphoblastic leukemia developed a severe but transient radiculopathy after intrathecal administration of methotrexate and cytarabine for an isolated central nervous system relapse. Chemotherapy was then given through an intraventricular catheter. Owing to a second central nervous system recurrence, she was treated with craniospinal radiation. The whole brain down to the level of C2 received a dose of 2400 cGy. Two months after completion of radiation, the child developed a progressive tetraparesis, and magnetic resonance imaging revealed an enhancing lesion involving the medulla and upper cervical cord. A biopsy was consistent with a treatment-related necrotizing leukoencephalopathy. This case suggests that patients who develop neurologic dysfunction when treated with methotrexate can also be particularly susceptible to radiation-related injury.