Vincristine, a widely used antineoplastic agent, is extremely toxic to the central nervous system. If given intrathecally, it produces a rapidly ascending, usually fatal, neuromyeloencephalopathy. We report a case of this complication in a 7-year-old girl with acute lymphoblastic leukemia who was receiving maintenance chemotherapy. During one treatment 0.5 mg of vincristine was erroneously injected into the lumbar subarachnoid space. Cerebrospinal fluid lavage was established within 2 h and continued for 24 h. After 7 days she developed a progressive sensorimotor paraplegia, which eventually stabilized as a paraparesis. Neurophysiological studies were consistent with an axonal type sensorimotor neuropathy. Magnetic resonance imaging of the spine was normal. Vincristine binds to cells, blocking mitosis, thus causing cell death. The associated central nervous system lesions are those of an ascending chemical leptomeningitis and ventriculitis. Cerebrospinal fluid lavage dilutes and removes the drug, thus limiting neural damage. At present this is the only treatment for intrathecal vincristine injection, and its early use in such an event is considered mandatory.