An 8-year-old child with acute lymphoblastic leukemia (ALL) developed seizures associated with bilateral lucencies on CT scan during standard induction therapy with vincristine and prednisone. Because of the progressive nature of her symptoms, a brain biopsy was performed and revealed neurotubular dissociation, which, in experimental animal models, is characteristic of vincristine toxicity. All other causes of encephalopathy have been ruled out. The drug was withheld and the clinical symptoms and findings resolved. The occurrence of unusual and severe neurologic side effects during the course of conventional induction therapy for childhood ALL warrants careful evaluation for evidence of vincristine toxicity.