Imuran Dosing in Crohn's Disease Study

Inclusion Criteria: * 10-70 years-old., Weigh 20-100 kg (44-220 lbs). * CD of the ileum, colon or ileocolon, verified by colonoscopy, barium enema, or small bowel series performed within 36 months * Perianal fistulae will be eligible provided that the perianal disease does not account for the preponderance of symptoms. * Have steroid-dependent, steroid-refractory or steroid naive CD. * Steroid-dependent CD: CDAI or mCDAI of \< 150 while receiving prednisone 10-40 mg/day or budesonide 3-9 mg/day for at least 12 weeks prior to screening, but unable to taper prednisone below 10 mg/day or budesonide below 3 mg/day without experiencing a flare within the previous 6 months. Steroids must be at a stable dose for 2 weeks prior to screening (week #-2), prednisone at a dose of 10-40 mg/day and budesonide at a dose of 3-9 mg/day. * Steroid-refractory CD: currently moderately active CD (CDAI or mCDAI 200 - 450) despite treatment with 40 kg) or 0.5 mg/kg/day (if weighing 20 mg/day (if weighing prednisone \<40 9 mg/day for the previous 4 weeks prior to the screening kg), or budesonide evaluation. Prednisone or budesonide must be at a stable dose for 2 weeks prior to screening (week #-2). * Steroid-naïve CD: currently moderately active CD, (CDAI or mCDAI 200 - 450) and one of the following: 1. Despite treatment with aminosalicylates and/or antibiotics for the previous 4 weeks prior to the screening evaluation, who are candidates for prednisone or budesonide. 2. Not currently on therapy, who are candidates for prednisone or budesonide 3. Patients with prior exposure to steroids, who have not been treated with steroids for 4 weeks prior to screening, and are candidates for prednisone or budesonide Prednisone or budesonide will be started at the screening visit, at a dose of 40 mg/day or 9 mg/day and tapered per the steroid taper. Patients who have started steroids up to 14 days prior to screening will also qualify as steroid naïve, however patient needs to be on 40 mg prednisone or 9 mg budesonide. * Discontinue oral or rectal 5-Aminosalicylic acid (5-ASA) therapies, rectal steroids, ciprofloxacin or metronidazole at the screening visit. Exclusion Criteria: * CDAI \> 450 * CD requiring hospitalization and intravenous (iv) corticosteroids, iv antibiotics or total parenteral nutrition (TPN). * TPN or enteral nutrition of \>1000 Calories/day (both TPN and elemental diets impact the CDAI). * History of resection of more than 100 cm of small bowel, total proctocolectomy, or subtotal colectomy with ileorectal anastomosis * Ileostomy or colostomy * Severe fixed symptomatic stenosis of the small or large intestine * Blood transfusion within 3 months before screening * Treatment with 6-MP or AZA within the 6 months prior to screening * Immunosuppressants or biologics 3 months before screening * Treatment 2 weeks before screening: * Allopurinol; * Trimethoprim-sulfamethoxazole; * NSAIDs or aspirin \>81mg/day; * Cholestyramine or other drugs interfering with enterohepatic circulation; * Furosemide and thiazide diuretics; * Fish-oil preparations. * Discontinue use at screening: Oral or rectal 5-ASA, rectal steroids, metronidazole or quinolones * Any prior treatment with natalizumab * Presence of abnormal laboratory parameters: * Carriage of hepatitis B surface antigen or positive hepatitis C antibody * Lack of one acceptable form of contraception while receiving AZA * Low TPMT activity