A patient had been treated for chronic lymphocytic leukemia for five years before developing visual blurring as the manifestation of occipital lobe lesions of progressive multifocal leukoencephalopathy (PML). The disease is caused by an infection of the CNS oligodendrocytes by a papovavirus, whose replication is facilitated by an impairment of the host's cell-mediated immunologic system. The multiplying virus destroys the oligodendrocytes and causes extensive demyelination of the white matter of the brain. Antiviral agents, such as cytarabine, may be beneficial in treating patients with PML.