Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system caused by the reactivation of a ubiquitous polyomavirus JC (JCV). PML was for many years a rare disease occurring only in patients with underlying severe impaired immunity. Over the past three decades, the incidence of PML has significantly increased related to the AIDS (acquired immunodeficiency syndrome) pandemic and, more recently, to the growing use of immunosuppressive drugs. The clinical presentation of PML is variable with neurological symptoms corresponding to affected cerebral areas. Usually, the clinical outcome of patients with PML is poor with an inexorable progression to death within 6 months of symptom onset. Although PML usually requires a brain biopsy or autopsy for confirmation, radiological imaging and a demonstration of JCV-DNA in the CSF (cerebrospinal fluid) provide supportive evidence for the diagnosis. Although there is no proven effective therapy for PML, patients with HIV (human immunodeficeincy virus)-related PML may benefit significantly from HAART (highly active antiretroviral therapy). In this article the author reviews the epidemiology, especially in Japan, current challenges in the diagnosis and the treatment guidelines of patients with PML based on recent advances in the understanding of the JC virus biology.