Progressive multifocal leukoencephalopathy (PML) developed in a 64-year-old woman who had undergone hemodialysis treatment due to chronic renal failure (CRF) for 6 yr. Her initial symptom was ataxia, and computed tomographic (CT) scan and magnetic resonance imaging (MRI) suggested a demyelinating disease of the cerebellum. Her cell-mediated immunity was impaired. At autopsy, the cerebellar disease was confirmed as PML by ultrastructural and immunohistochemical studies. Moreover, the JC type of papova virus infection was verified by Southern blot analysis.