A 48-year-old woman with a history of viral influenza infection was admitted with rapidly progressive numbness and weakness of the right extremities. On admission, general physical examination revealed no abnormality. Cerebrospinal fluid analysis showed no abnormal findings. Brain computed tomography and magnetic resonance imaging showed an open ring-like enhanced lesion in the white matter of the right parietal lobe with massive perifocal edema. Cerebral angiography showed no tumor staining and thallium-201 single photon emission computed tomography showed no abnormal uptake. The preoperative diagnosis was malignant glioma and partial resection was performed. Histological examination showed perivascular accumulation of small lymphocytes and a large number of macrophages with reactive astrocytes. Phagocytosis of myelin was observed in the macrophages and nuclear fragmentation in the reactive astrocytes. The histological diagnosis was acute inflammatory demyelinating disease. After therapy with methylprednisolone, her neurological symptoms improved gradually and no relapse occurred during 18 months of follow up. Tumor-like masses of demyelination may occupy an intermediate position between acute multiple sclerosis and postinfectious encephalitis. Open ring sign may be a pathognomonic feature of these lesions.