At present, there is no cure for MS. Based on the pathological concept that neuroinflammation is the common element leading or contributing to neurodegenerative changes, immune interventions have been introduced into clinical practice such as Natalizumab (Tysabri), a humanized monoclonal antibody. Natalizumab (Tysabri) is indicated as a disease-modifying monotherapy of highly active relapsing MS. The associated risks, especially progressive multifocal leukoencephalopathy, necessitate active monitoring of patients and a continuous discussion of optimum use of this drug. In clinical practice, the question how to manage patients on natalizumab at a higher risk for progressive multifocal leukoencephalopathy remains unresolved.