The authors report four adult patients with disseminated necrotizing leukoencephalopathy (DNL) following therapy for central nervous system (CNS) involvements of leukemia and lymphoma. Five to fourteen months after starting therapy with 30.6-62.5 Gy of whole brain irradiation and 120-500 mg of intrathecal methotrexate (MTX), DNL developed. Brain CT scans demonstrated a characteristic symmetrical white matter low density area. Furthermore, the brain CT scans disclosed tumorous lesions with contrast enhancement in three cases. In two of the three cases autopsy proved the tumorous lesions to be necrotic foci but not leukemic tumors. Post-mortem neuropathological studies of three patients disclosed characteristic features of DNL: multiple coagulative necrosis in the white matter with myelin degeneration, swollen axons, prominent calcification, and enlarged reactive astrocytes. Three of the four patients obviously received greater doses of whole brain irradiation and intrathecal MTX than patients who remained free from DNL after treatment with whole brain irradiation and/or MTX. Analysis of the etiological factors in this series underscores the prevailing danger of treatment for neoplastic CNS involvement with excessive doses of whole brain irradiation combined with intrathecal MTX.