A 44-year-old woman presented with dysarthria, visual disturbances, ataxia and cognitive impairment. There was a rapid progression of her neurological disease, and she died 8 months later. She was previously treated for a low-grade follicular B-cell lymphoma; complete remission was achieved by conventional radiotherapy and chemotherapy, including rituximab. Two years later, the neurological symptoms and signs started. MRI revealed a cerebral demyelinating process. Serology was negative. Autopsy disclosed areas in cerebral white matter with grey discoloration. Microscopy revealed demyelination, oligodendroglial viral inclusions and gliosis with bizarre astrocytes. Polymerase chain reaction (PCR) was positive for JC virus. These findings were consistent with progressive multifocal leukoencephalopathy (PML). This is one of recent reports on PML occurring in a patient treated with the anti-20 monoclonal antibody rituximab.