Crohn's disease, a chronic inflammation of the gastro-intestinal tract, is a disease with a complex pathogenetic background. Hereditary factors, environmental factors and the gut flora play a varying role in its onset, however, this role remains unclear to date. We hypothesize to detect tiny fragments of microbial DNA from the gut in the bloodstream, due to disruption in the natural defences of the gut lining. If this is true, it may hold the key to understanding why Crohn's disease recurs after surgery.

Our objective in this study is to test retrospectively, if we can detect any difference in microbial DNA from the blood of ileocolonic resection (ICR) patients that do and do not show recurrence at the time of follow-up endoscopy at 6 months.

Inclusion Criteria:

• Diagnosis of Crohn's Disease
• Adult ≥ 16 years
• Undergoing an ileocolonic resection or reresection
• Written informed consent

Exclusion Criteria:

• Use of antibiotics in the past 8 weeks at the timepoints when blood is withdrawn and feces is collected.
• Use of probiotics in the past 8 weeks at the timepoints when blood is withdrawn and feces is collected.
• Not willing to collect extra biosamples

Patients will be identified at the outpatient clinic at the dept. of gastroenterology or department of surgery or at a multidisciplinary meeting.