Progressive multifocal leucoencephalopathy was diagnosed by magnetic resonance imaging (MRI) and brain biopsy in a 44-year-old woman, for whom therapeutic trials with various antiviral agents were made. Despite early diagnosis and treatment, her neurological deterioration and extension of MRI-detectable lesions were not halted for a period of about 8 months. After the introduction of intrathecal beta-interferon therapy, her neurological status and MRI findings became stable and showed minimal improvement. Early diagnosis of this fatal disorder is important and intrathecal interferon therapy should be considered.