In case of primary progressive multifocal leukoencephalopathy, light microscopy demonstrated amphophilic intranuclear inclusion bodies, and electron microscopy confirmed the presence of papova-like and myxovirus in glial cells. The hypothesis is offered that the disease is caused by a slow virus, not necessarily having an altered immune mechanism as a prerequisite for its propagation, and that altered immunity may provide a more favorable background for its proliferation.