A case of acute lymphoblastic leukemia (ALL) which showed extensive demyelinated lesion of the brain stem during complete remission is presented. A 13 year-old girl who was diagnosed as ALL in February, 1987 was treated according to the TCCSG L84-11 protocol, which induced complete remission in June, 1987. Her condition had been stable until April, 1988 when she abruptly complained of gait disturbance, vertigo and anorexia. She was subsequently admitted to our hospital. At the time of admission, she had ataxic gait, horizontal nystagmus towards right at dextroversion, bilateral exaggerated patellar tendon reflex, emotional incontinence, urinary incontinence and left 6th and 7th nerve palsy. In the middle of May, right spastic hemipalesia and hypesthesia became apparent. Left caudal pontine-basal lesion was suspected and was confirmed by MRI. Her synptoms progressed. Five months after admission, suffered repeated central apnea and died. Autopsy disclosed extensive cervical spinal cord. Cerebrum and cerebellum were intact. No evident findings suggesting the etiology were obtained. Whole skull radiation, intrathecal methotrexate, cytosine arabinoside and hydrocortisone might have contributed to the development of the demyelinated lesion.