Progressive multifocal leucoencephalopathy is a serious side effect of natalizumab, a humanized monoclonal antibody approved for the treatment of multiple sclerosis . Here, we report a case of unexpected worsening of natalizumab-related progressive multifocal leucoencephalopathy following COVID-19 . After natalizumab discontinuation, a slight neurological improvement was observed, but, two months later the patient was admitted to the hospital because of neurological deterioration and COVID-19 mild pneumonia . Except for SARS-CoV-2 infection, no other potential factors of neurological worsening were identified . Thus, we pose the hypothesis that SARS-CoV-2 was instrumental in the progressive multifocal leucoencephalopathy deterioration.