A girl with chronic myeloid leukemia received bone marrow transplantation with total body irradiation as part of a conditioning regimen. She developed leukoencephalopathy about 50 days after transplant. There was permanent neurologic impairment afterward. She had not received any cranial irradiation or intrathecal chemotherapy before transplant. She had received 10 months of interferon treatment before transplant, which might have played a part in the development of leukoencephalopathy.