The purpose of this study is to determine the safety, tolerability, and efficacy of natalizumab in individuals diagnosed with active Crohn's Disease that are not in remission (CDAI greater than/equal to 150) and are currently taking Remicade. It is thought that natalizumab may stop the movement of certain cells, known as white blood cells, into bowel tissue. These cells are thought to cause damage in the bowel leading to the symptoms of Crohn's disease.

Patients who complete this study may be eligible for long-term natalizumab therapy via extension protocol ELN100226-351.

• Male and female patients at least 18 years of age with at least a six-month history of Crohn's disease and who are currently receiving Remicade and are not in remission (CDAI greater than/equal to 150)
• Women must not be breastfeeding or pregnant, and must not become pregnant during the study.