The primary objective of the study is to determine which baseline and yearly response factors (clinical and para clinical) predict overall disease-free status at Month 12 and Month 24, and clinical disease-free status in subsequent Months 36 and 48. The secondary objectives are: To identify prognostic factors at Baseline that predict overall disease-free status at Month 12, and to assess if yearly overall disease-free response factors predict overall disease-free status at Month 24; To evaluate clinical disease-free status (relapse, Expanded Disability Status Scale [EDSS]) at each analysis time point of Months 12, 24, 36, and 48; To identify prognostic factors at Baseline that predict clinical disease-free status at Month 12, and to assess yearly clinical disease-free response factors that predict clinical disease-free status (relapse, EDSS) in subsequent years at Months 24, 36, and 48; To evaluate the impact of Tysabri at each analysis time point of Months 12, 24, 36, and 48 on the following: annualized relapse rate (ARR), sustained EDSS progression and improvement (24-week sustained); To evaluate the impact of Tysabri at each analysis time point of Months 12, 24, 36, and 48 on the following: magnetic resonance image (MRI) measures: T2, T1, T1 with Gadolinium (Gd), brain atrophy; To evaluate the impact of Tysabri at Month 24 and Month 48 on the following: optical coherence tomography (OCT), Low and High Contrast Visual Acuity Assessment; To evaluate the impact of Tysabri at each analysis time point of Months 12, 24, 36, and 48 on the following: cognitive impairment (Symbol Digit Modalities Test [SDMT]), capacity for work (Work Productivity and Activity Impairment Questionnaire [WPAI]), quality of life (QoL) (Multiple Sclerosis Impact Scale [MSIS-29])

Key Inclusion Criteria:

• Documented diagnosis of Relapsing Multiple Sclerosis (McDonald 2010 Criteria ).
• <3 year disease duration.
• Must have an Expanded Disability Status Scale (EDSS) score from 0 to 4.0, inclusive.
• Anti-JCV antibody negative test within 6 months of Screening Visit.
• Must satisfy the approved therapeutic indications for Tysabri.
• Must be treatment-naïve to disease-modifying therapy (DMT) or have been treated with DMT (including but not limited to Avonex, Betaseron, Rebif, Copaxone, Extavia, or Gilenya) for ≤36 months total prior to date of informed consent.
• Decision to treat with Tysabri must precede enrollment.

Key Exclusion Criteria:

• Any prior treatment with Tysabri.
• Anti-JCV antibody positive at any timepoint prior to the Screening Visit.
• Contraindications to treatment with Tysabri as described in the US Prescribing Information.
• History of progressive multifocal leukoencephalopathy (PML) or other opportunistic infections, or an increased risk for such infections.
• History of diagnosis of Primary Progressive Multiple Sclerosis (PPM) and/or Secondary Progressive Multiple Sclerosis (SPMS).
• Receiving immunomodulatory or immunosuppressive therapy.
• Prior history of immunosuppressive use (mitoxantrone, azathioprine, methotrexate, cyclophosphamide, mycophenolate, cladribine, rituximab).
• Immunocompromised at the time of enrollment.
• Known active malignancies (subjects with cutaneous basal cell carcinoma that has been completely excised prior to study entry remain eligible).
• Women breastfeeding, pregnant, or planning to become pregnant; women who are not post-menopausal or surgically sterile who are unwilling to practice contraception.
• Inability to comply with study requirements.

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

Study participants will be recruited by participating neurologists from among their MS patients who opt to begin treatment with Tysabri.