The purpose of this study is to determine the safety and tolerability of natalizumab in adolescents (ages 12-17) diagnosed with moderately to severely active Crohn's disease (CD). 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 ELN100226-352.

Male and female patients, 12-17 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. Females must not be breastfeeding or pregnant, and must not become pregnant during the study.