An unusual cause of acute-onset and progressively worsening visual loss is presented. A 60-year-old woman was referred for left homonymous hemianopsia to our Emergency Medicine Department because of a suspected vascular accident. Ten years earlier she had been diagnosed as having chronic lymphocytic leukemia. Brain computed tomography and magnetic resonance imaging revealed "bilateral foci of white matter abnormalities in the occipital regions, compatible with a diagnosis of progressive multifocal leukoencephalopathy". Her cerebrospinal fluid was positive for papovavirus JC. Progressive multifocal leukoencephalopathy due to papovavirus JC, a typical complication in AIDS patients, is a rare complication in patients with other immunosuppressive conditions, such as chronic lymphocytic leukemia.