Natalizumab is a safe and efficacious treatment for inflammatory and autoimmune diseases, such as multiple sclerosis, Crohn's Disease, and rheumatoid arthritis. Rare occurrences of progressive multifocal leucoencephalopathy during treatment suggest the possibility that it may be related to natalizumab treatment. Monitoring for JCV and informing caregivers and patients about the manifestations of progressive multifocal leucoencephalopathy can improve the safety of natalizumab therapy.