The purpose of this study is to evaluate disease progression, in terms of development of symptomatic disease and complications associated with IBD (e.g. fistula, abscess, stricture).

This is a prospective, observational, multicenter, collaborative research project that will explore the earlier stages of IBD, before the onset of the first symptoms of the disease. This novel approach constitutes an innovative strategy on the research on the natural history of the disease. The study will be carried out based on colorectal cancer screening colonoscopies, recruiting all patients with a new diagnosis of IBD in this setting. These patients will undergo follow-up visits every 6 months for 10 years and the clinical information will be enriched with longitudinal multi-omic analyses.

Two additional control groups will be identified, including patients with new-onset symptomatic IBD in the last 3 months and healthy controls (with normal screening colonoscopy).

Inclusion criteria

Patients from cohort A will be included if they fulfill all the following inclusion criteria:

• Male or female ≥18 years of age at baseline.
• New diagnosis of IBD during a CRC screening colonoscopy, based on the criteria from the European Crohn's and Colitis Organization (33, 34).
• Presence of a chronic inflammatory infiltrate and histological diagnosis compatible with IBD.
• The patient must be asymptomatic at diagnosis and without previous symptoms suggestive of IBD.
• Time interval between the index colonoscopy and the baseline visit up to 3 months.

Patients from cohort B will be included if they fulfill all the following inclusion criteria:

• Male or female ≥18 years of age at baseline.
• Recent diagnosis of IBD, with <3 months from symptoms onset.
• Time interval between the index colonoscopy and the baseline visit up to 3 months.

Patients from cohort C will be included if they fulfill all the following inclusion criteria:

• Male or female ≥18 years of age at baseline.
• No endoscopic signs of IBD after a complete ileo-colonoscopy within the CRC screening program.
• Time interval between the index colonoscopy and the baseline visit up to 3 months.

Exclusion criteria

Patients from cohort A will be excluded if they fulfill any the following exclusion criteria:

• Identification of any enteropathogen in the stool culture.
• Isolated findings of acute inflammatory infiltrate without signs of chronicity.
• Previous or current diagnosis of microscopic colitis.
• Alteration in biomarkers in blood or stool will not constitute an exclusion criterion.

Patients from cohort B will be excluded if they fulfill any the following exclusion criteria:

- Previous use of immunomodulators or biologics for any condition.

Patients from cohort C will be excluded if they fulfill any the following exclusion criteria:

• Any gastrointestinal symptoms at baseline.
• Previous use of immunomodulators or biologics for any condition.

All patients with a new diagnosis of IBD