The purpose of this study is to determine the safety and efficacy of natalizumab in individuals diagnosed with moderately to severely active Crohn's disease. 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.

Male and female patients at least 18 years of age who have at least a six-month history of Crohn's disease and who are currently experiencing moderately to severely active Crohn's disease. Women must not be breastfeeding or pregnant, and must not become pregnant during the study.