Sponsor's decision.

The primary objective of the study is to determine the incidence and pattern of serious and/or clinically significant infections, malignancies, and other serious adverse event (SAE) in participants with Crohn's Disease (CD) treated with natalizumab. The secondary objective of this study in this study population is to evaluate disease severity over time in participants with CD treated with natalizumab based on changes in the Harvey-Bradshaw Index (HBI).

This study was originally conducted by Elan Pharmaceuticals, Inc. (Elan) in collaboration with Biogen under a protocol written by Elan. Biogen is solely responsible for the study since April 2013.

Key Inclusion Criteria:

• Subjects with Crohn's Disease (CD) who are eligible for therapy according to US Tysabri label and who are enrolled in the Tysabri Outreach Unified Commitment to Health (TOUCH) Prescribing Program.

Key Exclusion criteria:

• None

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

Participants with CD who were naïve to natalizumab treatment and participants with CD who have been previously treated with natalizumab.