The investigators intend to study vitamin D supplementation in children with Crohn's disease, ages 8 to 18 years. At the time of enrollment, the investigators will gather data on disease activity using both a simple history and physical exam, as well as blood and stool tests. In addition, the investigators will measure the levels of cathelicidin produced by the immune cells in their blood. The investigators will then supplement 20 children with vitamin D for a total of 6 months. During the study, patients will be seen every two months, where the investigators will monitor their vitamin D levels as well as perform rigorous safety monitoring for toxicity using blood and urine tests.