The primary objective of the study is to determine the association between prospectively measured disability (Expanded Disability Status Scale [EDSS]) with quality of life (Multiple Sclerosis International Quality of Life Questionnaire [MusiQoL]) at 3-month intervals up to 1 year in a real life setting of multiple sclerosis (MS) patients treated with natalizumab. The secondary objectives of this study are as follows: To evaluate the cumulative probability of sustained EDSS changes at 1 year following natalizumab treatment initiation; To evaluate the association between disability (EDSS), fatigue (Fatigue Scale of Motor and Cognitive Function [FSMC]), sexual dysfunction (Multiple Sclerosis Intimacy and Sexuality Questionnaire-19 [MSISQ-19]), depression (Beck Depression Inventory-Fast Screen [BDI-FS]) and neurocognitive function (Symbol Digit Modalities Test [SDMT]) with EuroQol-5D Questionnaire (EQ-5D) at 3 to 6-months intervals up to 1 year after initiation of natalizumab treatment; To assess the relationship between clinical disease-free status (no EDSS increase of 1.0 and no relapse) and MusiQoL at 1 year following natalizumab treatment initiation; To record the number of clinical relapses and relapses requiring steroid treatment at 3-months intervals up to 1 year after initiation of natalizumab treatment; To describe changes in work impairment (Work Productivity and Activity Impairment in MS [WPAI-MS]) at 3-months intervals up to 1 year after initiation of natalizumab treatment; To describe any change in the percentage of disability pension and occupation after 1 year of natalizumab treatment; To record the incidence and number of Serious Adverse Events (SAE) and Suspected Unexpected Serious Adverse Reactions (SUSAR) throughout the study

Key Inclusion Criteria:

• Subjects that have a confirmed diagnosis of RRMS, as per the 2010 revised McDonald criteria
• Subjects satisfying the therapeutic indication of natalizumab, as described in the local product label and confirmed by the Investigator. (Patients with previous treatment with natalizumab are also considered to be eligible only if the last natalizumab infusion was at least 1 year before the screening visit of this study)
• Decision for a treatment with natalizumab has been made before screening
• Patients with an EDSS score of 2.0-5.5 (both limits inclusive)

Key Exclusion Criteria:

• Patients with a diagnosed co-existing brain pathology other than MS, which in the judgement of the investigator impacts the value of EDSS or QoL.
• Pure spinal manifestation of demyelination
• Diagnosis of primary or secondary progressive MS
• Any change in concomitant medication known to affect cognition or bladder function
• A history of severe depressive disorder and/or suicidality, seizure, drug or alcohol abuse, as assessed by the Investigator

NOTE: Other Protocol Defined Inclusion/ Exclusion Criteria May Apply.

Participants diagnosed with relapsing remitting multiple sclerosis (RRMS) that have been prescribed natalizumab under standard clinical care.