The primary objective of this study is to assess the long-term safety and impact on disease activity and progression of Tysabri in participants with relapsing remitting multiple sclerosis (RRMS) in a clinical practice setting.

TOP is an epidemiological observational study of participants receiving natalizumab, with each participant to be followed for up to 15 years. This study is designed to address the long-term safety profile and the long-term impact on disease activity and progression of Tysabri with marketed use, and the impact of treatment on disability in particular by comparing the results with prospectively determined controls from established databases.

Key Inclusion Criteria:

• Documented diagnosis of Relapsing Remitting Multiple Sclerosis
• The decision to treat with Tysabri must precede enrollment
• Patient must be a new Tysabri user, and must not have had more than 3 Tysabri infusions prior to enrollment
• Must have had at least one relapse in the previous year, and must satisfy locally approved therapeutic indications for Tysabri

Key Exclusion Criteria:

• History of Progressive Multifocal Leukoencephalopathy or other opportunistic infections, or an increased risk of opportunistic infections
• History of positive anti-Tysabri antibodies
• Concomitant Immunomodulatory or immunosuppressive therapy during therapy with Tysabri
• Patient immunocompromised at the time of enrollment
• Known active malignancy
• Women must not be breast feeding or pregnant, or planning to become pregnant (must use birth control unless surgically sterile)

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.

Participants with RRMS who are therapy-naïve to Tysabri and who meet the criteria defined in the indication statement for prescription in the respective country.