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

iTOP is a retrospective and prospective Irish observational study of participants receiving natalizumab, with each participant to be followed for 3 years. This study is designed to address the long-term safety profile and the long-term impact on disease activity and progression of natalizumab with marketed use. Collection of efficacy and safety data at 6- monthly intervals to coincide with regular clinic visits and routine clinical practice will therefore be undertaken during the iTOP observational period.

Key Inclusion Criteria:

• Must give written informed consent and assent, as applicable.
• Decision to treat with natalizumab must precede enrollment.
• Patient characteristics and contraindications to treatment with natalizumab in accordance with prescribing information.
• Must be receiving natalizumab (Tysabri) for the treatment of RRMS in accordance with the natalizumab indication statement.
• Must have a documented diagnosis of Relapsing Remitting Multiple Sclerosis (RRMS).

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

Patients with RRMS who are being treated with natalizumab and patients who meet the criteria defined in the indication statement for prescription in Ireland. Existing patients will be enrolled retrospectively.