An 11-year-old boy developed a severe myelopathy after an eight-year history of acute lymphoblastic leukemia and numerous courses of intrathecal chemotherapy. Myelin basic protein in the cerebrospinal fluid (CSF) was elevated. Neuropathologic examination disclosed extensive microvacuolar changes in the white matter of the spinal cord. The pathogenesis of myelopathy following intrathecal chemotherapy administered by lumbar puncture includes an early effect on the myelin sheath. Serial assessment of CSF myelin basic protein levels in patients receiving intrathecal chemotherapy may be useful in the early diagnosis of this disorder.