Patients will be prescreened for inclusion criteria one week before the start of screening at Visit 0 (Prescreening Visit). Patients must be on stable doses of azathioprine/mercaptopurine. Patients will be given a diary to record their CD symptoms for the seven days prior to Visit 1. At Visit 1 (Screening Visit 1), patients will have their CDAI score assessed based upon their diary information. Patients with CDAI ≤ 220 will then have both calprotectin and hsCRP testing done. Patients with calprotectin > or = 250µg/g and/or hsCRP > or = 5mg/L will be notified and told to schedule Visit 2 within three weeks. At Visit 2 (Screening Visit 2), patients will undergo a colonoscopy. A Crohn's Disease Endoscopic Index of Severity (CDEIS) will be used to determine the endoscopic activity. Patients with significant endoscopic lesions will be notified and asked to enroll in the study.