Progressive multifocal leucoencephalopathy (PML) is a rarely occurring demyelinating disease of the central nervous system caused by a neurotropic papovirus named JC virus (JCV). The most frequently affected areas are the cerebral hemispheres, especially the parieto-occipital region, followed by the cerebellum and brain stem. The disease occurs predominantly in individuals with an immunocompromised state and impaired cellular mediated immunity (CMI) due to other underlying illness. The course of the disease is still most often rapidly progressive and fatal. Suspicion of PML should lead to an extensive immunological investigation before considering of brain biopsy, which is still the only specific test