Progressive multifocal leukoencephalopathy (PML) is considered so far as a rare rapidly progressive demyelinating CNS disease caused by the JC virus activation. Current data suggest that PML developed in people with impaired immunity. However, in some cases PML developed in the absence of severe immunological deficit. In clinical practice, the early diagnosis of PML is difficult because symptoms are mild and, therefore, the physician should be familiar with diagnostic criteria of this disease to make the correct diagnosis and start the symptomatic therapy as soon as possible. This will help to improve patient’s state though there is no effective treatment of PML so far. Up to the present moment, antiviral medications, cytostatic drugs, serotonin receptor antagonists, plasmapheresis are widely used. These methods can be used only if the causes of PML have been identified while the patient needs urgent help. Thus, the schemes of pathogenetic treatment with the drugs with pleiotropic effects should be used.