Progressive Multifocal Leukoencephalopathy (PML) is a demyelinating disease of the brain caused by the polyomavirus JC (JCV) in immunosuppressed people. There is no cure for PML but 1-year survival has increased from 10% to 50% in HIV-infected individuals treated with highly active antiretroviral therapy. We describe herein the clinical outcome of 24 PML patients whose survival exceeded 5 years, with a mean follow-up of 94.2 months (range, 60-188 months). Of all patients, only two were females including one who had non-Hodgkin's lymphoma and was HIV negative. All 23 HIV-positive patients received highly active antiretroviral therapy, and additional experimental therapies were not associated with a better clinical outcome. Marked neurological improvement occurred in 4/24 (17%) of patients, while 11/24 (46%) had partial improvement and 9/24 (37%) remained stable. By the end of the period of observation, 8/24 (33%) of patients had no significant disability despite persistent symptoms (modified Rankin disability scale (MRDS) =1), 6/24 (25%) had slight disability and were living independently (MRDS=2), 5/24 (21%) were moderately disabled, requiring some help during activities of daily living (MRDS=3) and 5/24 (21%) had moderately severe disability, requiring constant help or institutionalisation (MRDS=4). Patients with cerebellar lesions tended to have a worse clinical outcome. MRI showed leukomalacia with ventricular enlargement secondary to destruction of the white matter at the site of previous PML lesions, and focal areas of subcortical atrophy with preservation of the cortical ribbon. Of 20 patients tested, 19(95%) had detectable CD8+ cytotoxic T-lymphocytes against JCV in their blood. In absence of a specific treatment, immunotherapies aiming at boosting the cellular immune response against JCV may improve the prognosis of PML.