Abstract We report the case of a 34-year-old woman with clinical, neuroradiological and intraoperative histological findings, suggesting a low-grade astrocytic tumour. The demyelinating nature of the lesion was established through biopsy only after neurosurgery. The lesion size, in fact, great- ly exceeded that of the perivenous demyelination seen in typ- ical multiple sclerosis (MS) and tended to present as a space- occupying mass. This case underlines the importance of con- sidering demyelinating isolated lesions in the differential diagnosis of a brain mass. Since misdiagnosis can result in unwarranted and aggressive therapy, it is critical for the neu- rologist to be aware of this serious diagnostic pitfall. Key words Monofocal acute demyelinating lesions • Brain neoplasm Introduction The demyelinating diseases of the central nervous system (CNS) include several clinical entities, MS being the most common and best known. Whereas the neuroradiological pic- ture of MS is characterised by multiple demyelinating lesions separated from each other in space and time, acute, large, iso- lated white matter demyelinated lesions are encountered in a less defined group of diseases, usually affecting the cerebral hemispheres, and with radiological appearance and clinical course resembling a brain tumour [1]. The differential diagno- sis, in addition to neoplasm, includes Schilder’s disease, acute demyelinating encephalomyelitis, Balo’s concentric sclerosis, Marburg’s variant of MS, and brain infectious processes. Moreover, adrenoleukodystrophy, post-infection and post-vac- cination encephalomyelitis and progressive multifocal leuko- encephalopathy may rarely present as a mass lesion, while the concurrence of MS and brain tumour is exceptional [2]. Since misdiagnosis can result in unwarranted procedures [3], it is critical for the neurologist to be aware of this diag- nostic pitfall. Case report A 34-year-old healthy woman began to perceive rotation of the surroundings, with loss of postural control, and tendency to list towards the left. At examination, left deviation during the Unterberger test was observed, and a torsional nystagmus during Semont manoeuvre appeared in the II left position. Benign paroxysmal positioning vertigo was diagnosed and the patient was treated with particle repositioning. Because of the persistence of unsteadiness, the patient was submitted to neuroradiological investigations revealing a 20 mm cystic, space-occupying lesion, with surrounding edema, and locat- ed in the subcortical white matter of the right frontal lobe. The lesion was hypodense on CT scan, hypointense on T1 (Fig. 1a), and hyperintense on T2 and proton density MRI (Fig. Neurol Sci (2004) 25:S386–S388 DOI 10.1007/s10072-004-0349-6 J. Mandrioli • G. Ficarra • G. Callari • P. Sola • E. Merelli Monofocal acute large demyelinating lesion mimicking brain glioma J. Mandrioli • P. Sola • E. Merelli () Department of Neuroscience, Neurological Clinic University of Modena and Reggio Emilia Via Del Pozzo 71, I-41100 Modena, Italy e-mail: merelli.e@policlinico.mo.it G. Ficarra Department of Pathologic Anatomy and Legal Medicine Section of Pathologic Anatomy University of Modena and Reggio Emilia, Modena, Italy G. Callari Department of Neuroscience, Neuroradiology Unit University of Modena and Reggio Emilia, Modena, Italy