Adult individuals with ulcerative colitis in remission are given a single fecal microbiota transplantation (FMT) in colonoscopy. A half of the patients will receive autologous FMT as a placebo and a half will receive an FMT from a healthy donor.

80 adult patients with ulcerative colitis (UC) will be recruited. They are randomized to receive either an autologous FMT as a placebo or FMT from a healthy donor (frozen and thawed).

The patients are followed up with fecal samples to detect microbial changes. The follow up lasts 1 year after the FMT and a follow up colonoscopy will be done 52 weeks after the FMT.

Inclusion Criteria:

• • • Diagnosis of CU based on clinical, endoscopic, and histological findings.

  • Remission assessed by Mayo score (0 to 1) and F-Calpro (<100 ug/g).
  • Availability of consecutive fecal samples during one year after the diagnosis of CU.
  • Availability of blood sample to study the IBD associated genetic background
  • Compliance to attend ileocolonoscopy and FMT within 3-6 months after the diagnosis of CU and at 52 week
  • 18-75 years

Exclusion Criteria:

• Unable to provide informed consent
• Need for any antibiotic therapy within 3 months
• Use of corticosteroids, immunosuppressive or biological medication at the baseline
• Use of any probiotics
• Pregnancy

Individual participant data will not be shared.

The participant and the treating personnel are blinded of the randomization result.