Tysabri Observational Program

Key Inclusion Criteria:

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

Key Exclusion Criteria:

1. History of Progressive Multifocal Leukoencephalopathy or other opportunistic infections, or an increased risk of opportunistic infections
2. History of positive anti-Tysabri antibodies
3. Concomitant Immunomodulatory or immunosuppressive therapy during therapy with Tysabri
4. Patient immunocompromised at the time of enrollment
5. Known active malignancy
6. 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.