To compare treatment outcome in patients with Crohn's disease with secondary loss of response to infliximab (i.e. initial good response follow by loss of response) treated according to current standards based only on clinical features versus treatment based on serum levels of infliximab and anti-infliximab antibody (Ab) status.

Inclusion Criteria:

1. Patient must be able to understand the information given to him/her and give written informed consent.
2. Definitive diagnosis of Crohn's disease (confirmed by recent radiological, endoscopic and/or histological evidence according to international criteria) .
3. Age minimum 18 years.
4. Previous good response to at least 3 doses (5mg/kg) of infliximab (as judged by the treating physician).
5. Loss of response to standard doses of infliximab (as judged by the treating physician).
6. Last infliximab infusion given at least 4 weeks before inclusion.
7. For patients with luminal disease, the CDAI should be above 220 points at inclusion.
8. For patients with fistulising disease only, at least one draining perianal fistula (confirmed by radiography, MR, ultrasound or physical examination) should be present.

Exclusion Criteria:

1. Any contraindication to continued infliximab treatment
2. Short bowel syndrome
3. Bowel resection within 12 weeks of inclusion.
4. Current or recent history of severe, progressive, and/or uncontrolled renal, hepatic, haematological, gastrointestinal, endocrine, pulmonary, cardiac, neurological, or cerebral disease.
5. Pregnancy
6. History of alcohol or drug abuse within the prior year
7. Patients who do not meet concomitant medication criteria.
8. Any other condition, which in the Investigator's judgment would make the patient unsuitable for inclusion in the study.