 This study will compare two different dosing methods of azathioprine IMURAN in participants with Crohns disease who are currently taking steroids e.g. prednisone or budesonideor who have just started steroids. The study can be up to 54 weeks long. All participants enrolled will receive active drug. Participants will take doses either based upon weight or based on the patients ability to breakdown the drug monitored by 6-thioguanine nucleotides 6-TGN metabolite levels in the blood. All patients enrolled in the study will receive active study drug. 