To investigate whether sustained trough levels of IFX can be achieved using IFX (Infliximab) trough level measurements and adjustment of dosing based upon these levels by means of two different standardized algorithms in comparison with 'standard of care' IFX treatment and its effects on clinical and endoscopic outcomes.

Inclusion Criteria:

• Age > 18 years
• Active CD (CDAI>220) and signs of active inflammation as evidenced by elevated serum hsCRP levels (>5 mg/L) and/or elevated fecal calprotectin levels (>250 µg/g) and endoscopically visible ulcers.
• Patients must be naïve to biologics with indication for starting anti-TNF therapy in accordance with national reimbursement criteria.
• Patients must be naïve to thiopurines or have failed therapy with thiopurines (in which case AZA will be continued).
• Ongoing steroids are allowed if at stable dose for at least 2 weeks and at a maximum of prednisone 40 mg/d or budesonide 9 mg/day.
• Patients who consent to receiving Infliximab 5 mg/kg at week 0, 2 and 6 and further on every 8 weeks in conjunction with azathioprine (2,5 mg/kg/day). Patients who develop AZA intolerance during the trial are continued in the trial without AZA (ie IFX monotherapy).

Exclusion Criteria:

• Absence of endoscopically visible ulcers
• Prior exposure to infliximab (other biologics allowed)
• Ongoing steroid therapy at doses > 40 mg/d prednisolone equivalent
• Previous intolerance to azathioprine leading to drug discontinuation
• Ongoing infections
• Positive tuberculosis screen per local guidelines
• Serious other diseases including cancer in the 5 years prior to inclusion excluding non-melanoma skin cancer
• Indication for immediate surgery
• Pregnant or breast-feeding woman.
• Positive fecal culture for Salmonella, Shigella, Yersinia and Campylobacter and/or presence of Clostridium difficile B toxin in the stools
• Active tuberculosis
• Untreated latent tuberculosis (see national recommendations. Appendix 2).
• Non-compliant subjects.
• Participation in another therapeutic study.