A 9-year-old boy with acute lymphocytic leukemia underwent intrathecal methotrexate (MTX) treatment over a period of one year because of CNS symptoms. After a sudden rapid deterioration, radionuclide imaging demonstrated marked ventricular wall concentration, presumed related to MTX toxicity. After cessation of MTX treatment, the patient improved, and subsequent radionuclide images reverted to normal appearance.