We are reporting an autopsy case of so-called "acute multiple sclerosis" that was difficult to differentiate from a brain tumor on MRI findings. This case was a 69-year-old man, whose initial symptoms consisted of headache and unsteadiness in walking. Neurological findings included mild ataxia of the left upper extremity and positive Romberg sign. T 2-weighted MRI showed high intensity areas in the posterior limb of the right internal capsule and white matter near the posterior horn of the right lateral ventricle. Although the headache improved, the unsteadiness was exacerbated and the patient became unable to keep standing. Psychiatric symptoms and left hemiparesis were added to the clinical picture. The following MRI proved expansion of the previous lesions and the diffusely enhanced lesion spreading into the contralateral side through the corpus callosum. Stereotaxic biopsy showed the perivascular accumulation of small lymphocytes and a large number of bizarre astrocytes. Primary brain malignant lymphoma was diagnosed and radiation therapy was carried out. However, he developed perforation of the intestinal tract and died. Autopsy findings revealed scattered and disseminated small lesions in the cerebral white matter and the corpus callosum. There were a large number of lipid-laden macrophages, no stainable myelin and preserved axis cylinders in those lesions. Thus, those were interpreted as demyelinting lesions. They were scattered and multiple. This case was radiologically characterised by the diffusely enhanced, expanding butterfly-shaped lesion in bilateral cerebral hemisphere through the corpus callosum, and pathologically proven to be acute demyelination associated with severe perivascular infiltration of inflammatory cells. Multiple sclerosis may mimic neoplastic processes as trans-callosal hyperplastic neuroimage on neuroimaging like the present case.