Any prior treatment with Natalizumab.

Prior imunossupressive treatment (Mitoxantrone, Azathioprine, Methotrexate, Cyclophosphamide, Mycophenolate, Cladribine, Rituximab).

Contraindications to treatment with Natalizumab.

History of Progressive Multifocal Leukoencephalopathy (PML) or other opportunistic infections, or an increased risk for such infections.

Immunocompromised at the time of enrollment. Known active malignancies.

Inability to comply with study requirements.

Study of Tysabri in Early Relapsing Remitting Multiple Sclerosis Participants