Progressive multifocal leucoencephalopathy has become a growing concern in natalizumab-treated multiple sclerosis patients. Here, we describe a 35-year-old patient who was treated with 34 infusions of natalizumab before complaining about visual deterioration. MRI was non-diagnostic and JC virus testing initially was negative. Electroencephalography showed severe slowing of the right hemisphere, and neuropsychological testing revealed right frontal and temporal deficits. The diagnosis of progressive multifocal leucoencephalopathy was established 2 months later by typical MRI presentation and detection of JC virus DNA in the cerebrospinal fluid. Functional neurological deficits may precede imaging features and should prompt early consideration of progressive multifocal leucoencephalopathy.