Azathioprine in the Prevention of Ileal Crohn's Disease Postoperative Recurrence.

Inclusion Criteria: 1. Scheduled surgery for Crohn's disease (CD) with ileocolonic resection. Diagnosis of CD confirmed at least 4 months prior to surgery based on radiology and/or endoscopy. 2. Having an increased risk for postoperative relapse for any of the following reasons: (1) active inflammatory disease with C-reactive protein level (CRP) elevated above 10 mg/L, or the use of antibiotics, steroids or biological therapy including infliximab for active ileal disease within 2 months before surgery which were administered for Crohn's disease and not for another unrelated intercurrent inflammatory or infectious disease; (2) perforating disease defined as the presence of entero-enteric or enterocutaneous fistulas or perivisceral abscess formation within 2 months before surgery. Perianal fistulizing disease will not be considered as an indication of perforating disease in the ileum or right-sided colon. (3) Previous ileo-colonic resection, (4) active smoking, (5) age below 30. 3. Males and females 16-75 years old. 4. Curative surgical resection. All macroscopically inflamed colonic segments (except for anorectal involvement) are to be removed at surgery. Stricturoplasties in small bowel segments not involving the anastomotic region are allowed. 5. Patients able to start oral nutrition and oral therapy within 14 days from surgery. 6. Patients able and willing to give written informed consent 7. Women of childbearing potential should have a negative pregnancy test at inclusion. Exclusion Criteria: 1. Patients who only had strictureplasties or ileal/colonic resection without a new ileo-colonic anastomosis. Patients with ileorectal anastomosis. 2. Patients with no increased risk of postoperative relapse as defined before. 3. Patients with a known intolerance to azathioprine/6-mercaptopurine or with known homozygous thiopurine methyltransferase-low mutation. 4. Patients in whom more than 100 cm of small bowel has been previously resected. 5. Patients with active perianal disease or colorectal stenosis precluding ileocolonoscopy 6. Patients with sepsis or other postoperative complications necessitating use of antibiotics for more than 14 days after surgery. 7. Patients with liver test abnormalities (serum glutamate oxaloacetate transaminase, serum glutamate pyruvate transaminase, alkaline phosphatase, bilirubin \> 2 ULN), leucopenia (\<3000 white blood cell count /µL, \<1500 neutrophils /µL), thrombopenia with \< 50.000 platelets/mm3. 8. Patients with severe renal, pulmonary or cardiac disease. 9. Pregnant or lactating women. 10. Ongoing alcohol or substance abuse. 11. Ongoing or recent (within 6 months) infectious disease (viral hepatitis, tuberculosis, AIDS, Herpes zoster related disease). 12. Known malignancy 5 years from surgery except for superficial epithelioma of the skin with curative resection.